Fix numerous observed issues

* Fix crash in Ticket::getLastRespondent
* Fix crash is ORM if using a superclass of a relationship from cache. For
  instance, if the cache contains an ObjectThread instance, but a
  TicketThread instance is required in the relationship. In such a case, use
  the cached data, but change this class. This; however, violates the single
  nature of cache entries.
* Fix spurious events appearing after upgrade due to deleted tickets
* Ensure floating content in thread-body divs does not float past the bottom
* Provide an anchor to thread entries
* Show thread entries before events where both have the same timestamp
* Fix double events for collaborator additions
* Credit email sender with collaborator addition
* Fix incorrect rendering of collaborator addition event
* Fix crash on new ticket due to non-static function
* Fix missing sort on sub-query if explicitly declared in QuerySet
* Fix missing parentheses in SQL for ['field__eq' => QuerySet]
* Fix incorrect SQL on compileSelect for models with inherited $meta['table']
* Dynamically adjust relative time client side