This was the last thing still using burry.js, so we now have one less
dependency :)

- Use a template to show the day's date.
- Use time element for the date and datetime attr

Roster changes are listened to on the rosterview and the applicable chatbox is
updated.

Custom status messages are now remembered upon page load.

Also removed the need for the appendMessage method.

and do the fetching and sorting when the view is initialized

Specifically handle the case where a chatbox was destroyed, and then opened again.
We still have the view, so we reuse it.

Remove createChatBox from chatboxesview and move the message received listener to chatboxes

Move render, show and message fetching from ChatBoxesViews to ChatBoxView

and get rid of using chatboxesview to control chabox views

Couldn't save messages because localStorage wasn't activated for xmppchat.Messages

- Previous changes broke for the use-case with Plone, where the user
  doesn't log in manually.
- These changes probably broke the manual log-in use case again, but
  the design is now a bit cleaner and it won't be too hard to fix.
- Chatboxes are now also stored in backbone.localStorage \0/