[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.

I think that project_member and project_task_member should be one table - I don't see the reason to split them.
@somayjain I understand you suggested 2 tables - what's the reason for this?