* Rename old cascader
* Change name of old cascader
* Add basic cascader without search
* Add basic cascader without search
* Flatten options to make it searchable
* Add regex search and make backspace work
* Add barebone search without styles
* Add SearchResult list
* Add search navigation
* Rewrite of cascader and add some things to SelectBase
* Make SelectBase controlllable
* Cleanup
* Add initial value functionality
* Add onblur to hand caret direction
* New storyboom format for ButtonCascader
* Add knobs to story
* Add story and docs for UnitPicker
* Make UnitPicker use Cascader
* Fix backspace issue and empty value
* Fix backspace issue for real
* Remove unused code
* Fix focus issue
* Change children to items and remove ButtonCascaderProps
* Remove local CascaderOption
* Fix failed test
* Revert UnitPicker changes and change format for ButtonCascader
* Fix failing tests
* Prometheus: Display HELP and TYPE of metrics if available
- Prometheus recently added a metadata API around HELP and TYPE of
metrics
- request metadata when datasource instance is created
- use metadata to show help and type in typeahead suggestions and in
metrics selector as tooltip
* Fix types
- when instantiating a datasource, the datasource service checks if the
plugin module exports Explore components, and if so, attaches them to
the datasource
- Explore component makes all major internal pluggable from a datasource
`exploreComponents` property
- Moved Prometheus query field to promehteus datasource and registered
it as an exported Explore component
- Added new Start page for Explore, also exported from the datasource
