This was used to conditionally render messages in the group style, but
it's actually unnecessary. We can render the same markup in both cases
and change the appearance with css.
Move base64 encoding of attachments to an AttachmentView. This makes
image rendering an asynchronous task so we fire an update event to
indicate to the parent MessageListView that its content has changed
height and it is time to scroll down.
This dependency may be a little heavy for our current use case, but we can
roll with it for now and find something slimmer if it turns out yagni.
Closes#77Closes#40
Each conversation views now manages its own separate elements
rather than all binding to a shared #conversation element, and
similarly for message composition ui.
Also includes the beginnings of group creation UI (not working yet),
featuring bootstrap-tagsinput field for entering group recipients
