[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 usually try to learn by observing, atleast for coding conventions ;), maybe these need to be DRYed as well ?
https://github.com/flavour/eden/blob/master/modules/s3db/project.py#L6153