Server-side syntax highlighting for all code (#12047)

* Server-side syntax hilighting for all code

This PR does a few things:

* Remove all traces of highlight.js
* Use chroma library to provide fast syntax hilighting directly on the server
* Provide syntax hilighting for diffs
* Re-style both unified and split diffs views
* Add custom syntax hilighting styling for both regular and arc-green

Fixes #7729
Fixes #10157
Fixes #11825
Fixes #7728
Fixes #3872
Fixes #3682

And perhaps gets closer to #9553

* fix line marker

* fix repo search

* Fix single line select

* properly load settings

* npm uninstall highlight.js

* review suggestion

* code review

* forgot to call function

* fix test

* Apply suggestions from code review

suggestions from @silverwind thanks

Co-authored-by: silverwind <me@silverwind.io>

* code review

* copy/paste error

* Use const for highlight size limit

* Update web_src/less/_repository.less

Co-authored-by: Lauris BH <lauris@nix.lv>

* update size limit to 1MB and other styling tweaks

* fix highlighting for certain diff sections

* fix test

* add worker back as suggested

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/dlclark/regexp2/syntax/replacerdata.go

@@ -0,0 +1,87 @@
+package syntax
+
+import (
+	"bytes"
+	"errors"
+)
+
+type ReplacerData struct {
+	Rep     string
+	Strings []string
+	Rules   []int
+}
+
+const (
+	replaceSpecials     = 4
+	replaceLeftPortion  = -1
+	replaceRightPortion = -2
+	replaceLastGroup    = -3
+	replaceWholeString  = -4
+)
+
+//ErrReplacementError is a general error during parsing the replacement text
+var ErrReplacementError = errors.New("Replacement pattern error.")
+
+// NewReplacerData will populate a reusable replacer data struct based on the given replacement string
+// and the capture group data from a regexp
+func NewReplacerData(rep string, caps map[int]int, capsize int, capnames map[string]int, op RegexOptions) (*ReplacerData, error) {
+	p := parser{
+		options:  op,
+		caps:     caps,
+		capsize:  capsize,
+		capnames: capnames,
+	}
+	p.setPattern(rep)
+	concat, err := p.scanReplacement()
+	if err != nil {
+		return nil, err
+	}
+
+	if concat.t != ntConcatenate {
+		panic(ErrReplacementError)
+	}
+
+	sb := &bytes.Buffer{}
+	var (
+		strings []string
+		rules   []int
+	)
+
+	for _, child := range concat.children {
+		switch child.t {
+		case ntMulti:
+			child.writeStrToBuf(sb)
+
+		case ntOne:
+			sb.WriteRune(child.ch)
+
+		case ntRef:
+			if sb.Len() > 0 {
+				rules = append(rules, len(strings))
+				strings = append(strings, sb.String())
+				sb.Reset()
+			}
+			slot := child.m
+
+			if len(caps) > 0 && slot >= 0 {
+				slot = caps[slot]
+			}
+
+			rules = append(rules, -replaceSpecials-1-slot)
+
+		default:
+			panic(ErrReplacementError)
+		}
+	}
+
+	if sb.Len() > 0 {
+		rules = append(rules, len(strings))
+		strings = append(strings, sb.String())
+	}
+
+	return &ReplacerData{
+		Rep:     rep,
+		Strings: strings,
+		Rules:   rules,
+	}, nil
+}