Add the idea of a "parent message" to Messages, used for Replying

* You should be able to reply to a message. This means that we have to
  store which message you are replying to. These will be changed when
  showing a particular message.
Vendor stuff.
Add actions to reply to messages, show parent messages in chain

* Can now reply to specific messages using the "reply" form.
* When viewing a message, its parents are chained (the message it's in
  reply to is rendered, along with the message the parent replied to,
  until there are no more parents left).
Use laravel form generators to restore input when writing new message
Show who the message will be sent to when writing a reply.
Can click on the subject of a message to get a link to it.