E1904 Issues related to topic deadlines

The following modifications were made so far:

1. Copied in some code from the previous semester's implementation of this project (they didn't do it properly and weren't permitted to check-in)

2. Successfully recreated Expertiza issue #98

3. Added field auto_dropped_from_waitlist to SignedUpTeam

4. When checking whether a team needs to be removed from the waitlist, looped through all waitlisted teams on all topics for which the drop topic deadline has passed

5. Renamed the update_is_waitlisted method to clear_waitlisted_teams_for_topic to better reflect what the method does

6. Verified Expertiza issue #98 has been fixed

7. Tested to ensure things work if it’s a staggered deadline assignment

8. Updated documentation on wiki

Still yet to complete:

1. Refactor code implementation for fixing of Expertiza issue #98

2. Fix Expertiza issue #154

Accounts used for testing:

- instructor6/password

- student6390 (can impersonate using instructor6 account)

- student6391 (can impersonate using instructor6 account)

- student6392 (can impersonate using instructor6 account)

- student6393 (can impersonate using instructor6 account)

- student6394 (can impersonate using instructor6 account)

- student6395 (can impersonate using instructor6 account)

- student6396 (can impersonate using instructor6 account)

Other related issues found:

- When editing an assignment's due dates, the drop topic deadline won't display unless the signup deadline is displayed

- When a student is on the waitlist for more than one topic, only the row for one of the topics is grey

- There seems to be about a 20 minute drop topic deadline lag, some of the time