Refs #403 Allowed to follow boards

 - Added schema for boards_followers with constraints to enforse data integrity
 - Added API method which allows to follow/unfollow a given board
 - Added view which passes request to an API method
 - Added JS handler for follow/unfollow board clicks
 - Cleaned up "Add board" buttons from several templates
After code review fixes. refs #403
Merge pull request #7 from OlegYarosh/board_follows

Board follows