Previously the conversation window would query the background page
for a model id and then fetch the conversation. Instead, we can fetch
the conversation before opening the window, which simplifies the front
end scripts and avoids creating multiple copies of the same model.
When a new message arrives, if its conversation is not already opened,
the background page opens it. If it is alrady open the window is
focused. Finally, the 'message' event is triggered, resulting in
1. the inbox refetches conversations
2. all conversations fetch new messages
TODO: only send this event to the target window
Render the entire conversation from a template, because some parts of it
must be rendered conditionally if it is a group vs private conversation.
Also apply some style fixes and restore lost functionality:
* Make conversation title bar fixed.
* Widens message bubbles.
* Unhide message list.
* Restore attachment rendering.
* Restore message sending and attachment file selection.
* Style attachments file input as a paperclip.
* Style send button like on Android and make it a submit input.
