[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.

@michaelhowden : I suggested 2 tables when the requirement included user-defined roles. But, if we are just keeping an interested option, then there is no need for 2 tables.