[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.

NB, you could just write:

    s3.prep = lambda r: r.method == "options" and r.representation == "s3json"

(but only if you want - your variant is totally fine)

;)