[SSF] Members for tasks

Added an "Interested" field that would show up in the rheader/form for Tasks, and users can 'Watch' a task(i.e, assign themselves a 'following' role). However, deployment settings can be changed to allow Users to assign themselves other roles such as "Mentor", "Volunteer" that shows up as a dropdown-list along with the "Watch" button.

Btw - it is not 100% clean to respond to invalid format and invalid method with the same HTTP status code. An invalid format is responded to with a 501, but an unsupported method is actually a 405 - and method should take precedence over format.