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
* Karma to Jest: datasource_srv
* Sort function differs between Karma and Jest
* Fix error based on .sort() implementation
* Remove Karma test
* alerting: only log when screenshot been uploaded
* Remove comments
* Karma to Jest: datasource_srv
* Sort function differs between Karma and Jest
* Fix error based on .sort() implementation
* Remove Karma test
* Remove comments
* Remove console.log
* Remove console.log
* Change sorting and add test for default data source
* webpack poc, this is not going to work for plugins, dam
* tech: webpack and systemjs for plugins starting to work
* tech: webpack and systemjs combo starting to work
* tech: webpack + karma tests progress
* tech: webpack + karma progress
* tech: working on tests
* tech: webpack
* tech: webpack + karma, all tests pass
* tech: webpack + karma, all tests pass
* tech: webpack all tests pass
* webpack: getting closer
* tech: webpack progress
* webpack: further build refinements
* webpack: ng annotate fixes
* webpack: optimized build fix
* tech: minor fix for elasticsearch
* tech: webpack + ace editor
* tech: restored lodash move mixin compatability
* tech: added enzyme react test and upgraded to react v16
* tech: package version fix
* tech: added testdata to built in bundle
* webpack: sass progress
* tech: prod & dev build is working for the sass
* tech: clean up unused grunt stuff and moved to scripts folder
* tech: added vendor and manifest chunks, updated readme and docs
* tech: webpack finishing touches
