General cleanup of CSS, which was getting disorganized. Now each page's container div has an id of "controller-action" (i.e. "bounties-new") so CSS can be written that affects only a specific page, prevent styles for one page from affecting another.
Rename the error_messages partial to bounty_error_messages to be clearer what model type it expects (it's currently being mistakenly used in artist views and generating errors).
Add documentation to the _bounty_error_messages partial.
Add a partial for rendering error messages on artist models.
The bounty filters now always apply the viewable_by filter for the current user.