[#6831] Allow standard browser session to auth REST API

Signed-off-by: Cory Johns <admin1@users.sf.net>
[#6831] Added API controller for ForgeActivity

Signed-off-by: Cory Johns <admin1@users.sf.net>
[#6831] Added RSS/ATOM feed to Activity Stream and refactored common code

Signed-off-by: Cory Johns <admin1@users.sf.net>
[#6831] Minor fixes

- No trailing slash on feed urls
- Not all activity_extras dicts will have a 'summary' key

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>