[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.

No, the principle is to use different methods for different actions - "assign" to assign and "remove" to remove the role. Can both be handled by the same method handler, however - just configure like:

    s3db.set_method("project", "task", method="assign", action=assign_role)
    s3db.set_method("project", "task", method="remove", action=assign_role)

and then check for:

    if r.method == "assign":
        # Code for assignment
    elif r.method == "remove":
        # Code to remove assignment