History and Version Control for Dashboard Updates

A simple version control system for dashboards. Closes #1504.

Goals

1. To create a new dashboard version every time a dashboard is saved.
2. To allow users to view all versions of a given dashboard.
3. To allow users to rollback to a previous version of a dashboard.
4. To allow users to compare two versions of a dashboard.

Usage

Navigate to a dashboard, and click the settings cog. From there, click
the "Changelog" button to be brought to the Changelog view. In this
view, a table containing each version of a dashboard can be seen. Each
entry in the table represents a dashboard version. A selectable
checkbox, the version number, date created, name of the user who created
that version, and commit message is shown in the table, along with a
button that allows a user to restore to a previous version of that
dashboard. If a user wants to restore to a previous version of their
dashboard, they can do so by clicking the previously mentioned button.
If a user wants to compare two different versions of a dashboard, they
can do so by clicking the checkbox of two different dashboard versions,
then clicking the "Compare versions" button located below the dashboard.
From there, the user is brought to a view showing a summary of the
dashboard differences. Each summarized change contains a link that can
be clicked to take the user a JSON diff highlighting the changes line by
line.

Overview of Changes

Backend Changes

- A `dashboard_version` table was created to store each dashboard
  version, along with a dashboard version model and structs to represent
  the queries and commands necessary for the dashboard version API
  methods.
- API endpoints were created to support working with dashboard
  versions.
- Methods were added to create, update, read, and destroy dashboard
  versions in the database.
  - Logic was added to compute the diff between two versions, and
  display it to the user.
  - The dashboard migration logic was updated to save a "Version
  1" of each existing dashboard in the database.

Frontend Changes

- New views
- Methods to pull JSON and HTML from endpoints

New API Endpoints

Each endpoint requires the authorization header to be sent in
the format,

```
Authorization: Bearer <jwt>
```

where `<jwt>` is a JSON web token obtained from the Grafana
admin panel.

`GET "/api/dashboards/db/:dashboardId/versions?orderBy=<string>&limit=<int>&start=<int>"`

Get all dashboard versions for the given dashboard ID. Accepts
three URL parameters:

- `orderBy` String to order the results by. Possible values
  are `version`, `created`, `created_by`, `message`. Default
  is `versions`. Ordering is always in descending order.
- `limit` Maximum number of results to return
- `start` Position in results to start from

`GET "/api/dashboards/db/:dashboardId/versions/:id"`

Get an individual dashboard version by ID, for the given
dashboard ID.

`POST "/api/dashboards/db/:dashboardId/restore"`

Restore to the given dashboard version. Post body is of
content-type `application/json`, and must contain.

```json
{
  "dashboardId": <int>,
  "version": <int>
}
```

`GET "/api/dashboards/db/:dashboardId/compare/:versionA...:versionB"`

Compare two dashboard versions by ID for the given
dashboard ID, returning a JSON delta formatted
representation of the diff. The URL format follows
what GitHub does. For example, visiting
[/api/dashboards/db/18/compare/22...33](http://ec2-54-80-139-44.compute-1.amazonaws.com:3000/api/dashboards/db/18/compare/22...33)
will return the diff between versions 22 and 33 for
the dashboard ID 18.

Dependencies Added

- The Go package [gojsondiff](https://github.com/yudai/gojsondiff)
  was added and vendored.

vendor/github.com/sergi/go-diff/diffmatchpatch/diffmatchpatch.go

@@ -0,0 +1,46 @@
+// Copyright (c) 2012-2016 The go-diff authors. All rights reserved.
+// https://github.com/sergi/go-diff
+// See the included LICENSE file for license details.
+//
+// go-diff is a Go implementation of Google's Diff, Match, and Patch library
+// Original library is Copyright (c) 2006 Google Inc.
+// http://code.google.com/p/google-diff-match-patch/
+
+// Package diffmatchpatch offers robust algorithms to perform the operations required for synchronizing plain text.
+package diffmatchpatch
+
+import (
+	"time"
+)
+
+// DiffMatchPatch holds the configuration for diff-match-patch operations.
+type DiffMatchPatch struct {
+	// Number of seconds to map a diff before giving up (0 for infinity).
+	DiffTimeout time.Duration
+	// Cost of an empty edit operation in terms of edit characters.
+	DiffEditCost int
+	// How far to search for a match (0 = exact location, 1000+ = broad match). A match this many characters away from the expected location will add 1.0 to the score (0.0 is a perfect match).
+	MatchDistance int
+	// When deleting a large block of text (over ~64 characters), how close do the contents have to be to match the expected contents. (0.0 = perfection, 1.0 = very loose).  Note that MatchThreshold controls how closely the end points of a delete need to match.
+	PatchDeleteThreshold float64
+	// Chunk size for context length.
+	PatchMargin int
+	// The number of bits in an int.
+	MatchMaxBits int
+	// At what point is no match declared (0.0 = perfection, 1.0 = very loose).
+	MatchThreshold float64
+}
+
+// New creates a new DiffMatchPatch object with default parameters.
+func New() *DiffMatchPatch {
+	// Defaults.
+	return &DiffMatchPatch{
+		DiffTimeout:          time.Second,
+		DiffEditCost:         4,
+		MatchThreshold:       0.5,
+		MatchDistance:        1000,
+		PatchDeleteThreshold: 0.5,
+		PatchMargin:          4,
+		MatchMaxBits:         32,
+	}
+}