- Fix memory leaks when removing a vhost request from the outstanding
  requests list. Also consolidate the code responsible for this into
  one function, to save having to repeat it 6 times.

- Add a silent rejection feature. If a rejection reason is provided,
  and it is the string "SILENT", no memos or notices will be sent to
  the user whose vhost is being rejected.

- Add an option restricting users from making a subsequent vhost
  request until they no longer have an outstanding request that has
  yet to be accepted or rejected.