* [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
* move to explore folder and delete explorev2 folder
* add tests for fetchDatasourceMetadata
* tests for fetchDatasources, fetchDatasourceMetadata
* use $.ajax for fetch dashboards and write test
