End states for mentorship requests

[ecxia]

Closes #124. Sorry this one is a bit bulky - some of the cleanup stuff should have been added to a different pull request.

Request detail template:

• Updates to include ability to complete requests. Either party to an accepted request can unilaterally do this.
• Also creates portion of template shown for completed request

Request list template:

• Updates request list to show badges for completed requests.
• Also cleans up request lists so all have uniform behavior (not showing ignored requests)

Models:

• Adds COMPLETED to list of possible statuses
• In save method, reshuffles local variables & context sent to email so can send two emails (one to requestor, one to requestee) in a single call
• Updates save method so that when request status is set to COMPLETED, sends emails notifying both requestor and requestee.

Email templates:

• Creates base "mentorship completed" email template and extends for requestor and requestee

Tests

• Slimmed down setup a bit by using create_test_users function instead of creating all objects associated with a user separately.
• Created unit test for request completion behavior (request & offer versions)
• Created unit tests for complete request view

views, URLs

• added "complete request" view

[rifferreinert]

non blocking but could we change these to check against the static named variables instead of numbers?