When first intalling, users will no longer be presented with the option
to register as a standalone client.
For developer convenience, the standalone form can still be found at
chrome-extension://.../register.html
Closes#159
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.
