Creates user-bar many to many relationship through bar_gig_memberships

* Creates owner-owned bar relationship
* Generates bar_gig_membership model
* Adjusts partials to display new associations
Hmm.  Good point.  I don't actually enforcement by either of those methods.  Instead, when a bar is created, I automatically merge in the current_user's id into the user_id column in the bar table.  I should probably at least add a validation.