Support End States for Requests

[raymondberg]

Problem

If a mentorship continues for 3 months and ends naturally (or otherwise), the state of the pairing is no longer
the same as when it was Requested, nor when it was Accepted. It is in a new state of Completed or
similar language.

Proposal

Allow for a Mentor or Mentee to change the state of an Accepted request to Completed. In doing so,
Completed requests no longer count to capacity outlined in #122, and the mentor will show up in #123
if they are now under their ideal capacity.

[ZaxR]

This ticket will focus on just adding a new state for completed. The automatic management vs capacity will be added in #128