* fix DataSourceAddButton component
This component implicitly returned `false` when `canCreateDataSource`
was false, and that way it didn't qualify as a function component.
We fixed it to explicitly return null in this case.
* DataSourceAddButton: update function signature
This is more consistent with other code in this codebase.
* navtree.go: update Data sources title and subtitle
* DataSourceList: move add button to header
* DataSourcesList: add buttons to items
The action buttons are added inside `<Card.Tags>` so that they end up at
the right end of the card, as it was designed.
The "Build a Dashboard" button's functionality is not defined yet.
* DataSourcesListHeader: add sort picker
* fix css
* tests: look for the updated "Add new data source" text
* tests: use an async test method to verify component updates are wrapped in an act()
* update e2e selector for add data source button
* fix DataSourceList{,Page} tests
* add comment for en dash character
* simplify sorting
* add link to Build a Dashboard button
* fix test
* test build a dashboard and explore buttons
* test sorting data source elements
* DataSourceAddButton: hide button when user has no permission
* PageActionBar: remove unneeded '?'
* DataSourcesList: hide explore button if user has no permission
* DataSourcesListPage.test: make setup prop explicit
* DataSourcesList: use theme.spacing
* datasources: assure explore url includes appSubUrl
* fix tests and add test case for missing permissions
Co-authored-by: Levente Balogh <balogh.levente.hu@gmail.com>
