* Allow for multiple color schemes
1. create ColorSchemeControl component
2. using the same new control component for linear colors
* add color spectum for linear color scheme
* remove dup css
* fix controls setting for linear color scheme
* minor fix by code review comment
* [explore] DatasourceControl to pick datasource in modal
Makes it easier to change datasource, also makes it such that the list
of all datasources doesn't need to be loaded upfront.
* Adding more metadata
