* Web interface to add and delete data sources, without the need to ssh
into the server.
* Ability to safely delete datasources -- query results from this data sources
are deleted, while queries get assigned null datasource.
* Updated the BigQuery datasource to use the JSON key file from Google Developer
console. Also both BigQuery and the Google Spreadsheets datasource no longer store
their key on the filesystem, but rather in the DB.
* Minor updates to the Flask Admin.
