[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.

Not really - the *correct* way would actually be to require HTTP PUT to assign a role, and HTTP DELETE to remove the assignment. But most browsers do neither support the one nor the other, so you only have HTTP POST - and the Eden way out of this is to send to different sub-resources (=URL "methods") for different actions.

Surely you can qualify the method further by using a query variable - but for consistency reasons, we should use the URL "method" pattern (because we do that elsewhere).