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
Yes, as currently set up, there must be an owner when a bar is created.  I was thinking about adding the allow_nil option, but I wasn't sure if this would be considered good coding practice if a bar cannot currently be created without an owner.  Thoughts?