RSS for branches

As a Reader I'd like to have per-branch RSS because I'm pretty selective and don't want to read everything

Acceptance criteria:

1. There is an icon (same as rss icon on main page) on branch page above topic list (on the right side)
2. User opens rss feed by clicking on icon
3. When user opens rss feed first time - she receives last 15 messages
4. User should not receive deleted messages
5. RSS should be autodiscovered by browsers
6. Posts should appear in rss feed according to VIEW_TOPIC permission for branch:
   • if user is not logged in - we should use permissions for "Anonymous" user group
   • if user logged in - we should use her permissions