Change browser feed metadata on project and context pages to point to a feed for that project/context

[dnrce]

Migrated from the original issue at https://www.assembla.com/spaces/tracks-tickets/tickets/1459

While Tracks supports per-project and per-context RSS/iCal feeds, these feeds are not linked in the page metadata for project and context views. Clicking the Firefox 'RSS' icon on these pages takes the user to the global feed.

Rather than hiding more specific feeds behind the Feeds menu, they should be on their respective pages. The RSS feed for a page should contain the information on that page, or not be present if such a view is impossible. This creates a more consistent user experience.

Originally reported by tedks on April 4, 2014 at 23:56:51 (+0800) against version git-master