Adds a datasource selector to the Explore UI. Only datasource plugins
that have `explore: true` in their `plugin.json` can be selected.

- adds datasource selector (based on react-select) to explore UI
- adds getExploreSources to datasource service
- new `explore` flag in datasource plugins model
- Prometheus plugin enabled explore

Fix freezing browser when loading plugin

- broken since 4d2dd220
- `*` was previously working as a path matcher, but freezes browser when
  used with new cache-busting plugin loader
- changed matcher to be `/*`

* check extended statistics pattern

* check extended statistics pattern

* Revert "check extended statistics pattern"

This reverts commit 52c7b1a972636d5f5729e64ae5e00e6fae329257.

* add test

* fix test

* Rewriting team pages in react

* teams to react progress

* teams: getting team by id returns same DTO as search, needed for AvatarUrl

* teams: progress on new team pages

* fix: team test

* listing team members and removing team members now works

* teams: team member page now works

* ux: fixed adding team member issue

* refactoring TeamPicker to conform to react coding styles better

* teams: very close to being done with team page rewrite

* minor style tweak

* ux: polish to team pages

* feature: team pages in react & everything working

* fix: removed flickering when changing tabs by always rendering PageHeader

* prevent error to use $__interval_ms in query

* add test

* prevent error to use $__interval_ms in query

fix: folder picker did not notify parent that the initial folder had been changed, fixes #12543 (#12554)

* New rebase

Signed-off-by: Lukasz Gryglicki <lukaszgryglicki@o2.pl>

* Lint

Signed-off-by: Lukasz Gryglicki <lukaszgryglicki@o2.pl>

* Add folder name to dashboard title. No display on medium displays and below

* Compare folderId instead

* folderId bigger than 0

- allow css loader to be imported again (wasnt prefixed by plugin)

* Add new sequential color scales

* Simplify inversion logic

Karma to Jest: 3 data sources

Query field refactorings to support external plugins

* make diff and percent_diff tests more realistic

* fix diff and percent_diff

* include @marefr's additional tests

* tabs to spaces

testing commit permisions :)

* revert

* Update lodash and moment

* tabs to spaces

testing commit permisions :)

* revert

* tabs to spaces

ux: minor fix/tweak to inactive view mode, think logo should be visible & fixes dashboard title alignment

[skip ci]

Fix datasource sorting with template variables

- fixes sorting when template variables are present
- simplified existing test cases
- added test to cover variable usage in datasource service

[skip ci]

[skip ci]

Prometheus heatmap: fix unhandled error when some points are missing

[skip ci]

Fix links not updating after changing variables

mysql,mssql: improve $__timeFilter, $__timeFrom, $__timeTo macros

The $__timeFilter macro now uses a BETWEEN with timestamps instead of 
comparing epochs
The $__timeFrom and $__timeTo macros now return strings of format 
YYYY-MM-DDThh:mm:ssZ instead of epoch