Add Flask-Migrate support

Add Flask-Migrate to manage database schema changes
Merge pull request #9 from armooo/flask-migrate

Add Flask-Migrate support
Normalise quotes to single quotes
Add group statuses

Add statueses to groups and default new groups to have the status of
proposed.
Filter study groups based on proposed

Do not list proposed groups to non-admins. Also respond with a 404 to
groups that are proposed to non-admins.
Add Flask-Migrate support

Add Flask-Migrate to manage database schema changes
adding group
adding membership
adding tests
adding messaging on group join
no leader no message
adding support for a list of leaders
trying to merge things
merging active status and adding migration for missing tables