Featured datasets

[michellethomas]

Hi,

This creates a page for featured datasets.

Major changes:
-Adding featured datasets page and corresponding css stylesheet
-Adding is_featured to Sqlatable and Datasource models and creating a database migration for the upgrade.
-Adding user_id as an owner to Sqlatables, creating a relationship with the ab_user table, and creating a database migration for the upgrade
-Adding both is_featured and owner into the views
-Adding a link to featured datasets into the navbar

Notes:
-The database migration for user_id in Sqlatables was a little complicated. It seems that Alembic has problems with multiple alter table statements in one update, so I had to add batch_alter_tables, and it also defaulted to having no name for the constraint (which threw an error so I named it "user_id".
-Spent a while fixing the downgrade function for the sqlatables user_id db migration. I believe because I'm batch altering a table, it already has the table_name, so alembic doesn't need it when I'm calling drop_constraint and drop_column
-It looks like in views.py appbuilder shows the links in the order that you add them to the file. If we want to add featured datasets to the top, I had to put the add_link in at the top of the file, away from the featured_datasets function.
-Ran run_tests.sh and passed

Screenshot: