Problem report should still show the name of a deleted user

[chrisjwelly]

In #346 , I have added a placeholder "(Deleted member)" if a problem report is made by a user which has been deleted from the database.

Ideally, even if the user has been deleted, we should still be able to show the user's name (perhaps with a tag that he or she is an old member so as not to confuse the new member).

A quick dirty fix is to save the reporter's name in the Problem Report model, but this may not be ideal as there will be a change in the database..

Not critical as this problem will only occur for a new academic year. We should also at least add tests for this.