Add contentType-specific limits, switch to lazy-init iff we encounter an
oversized file, and restyle as a toast, factoring out a generic
ToastView along the way.
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.
