Previously, the ugly file input was hidden with opacity, and styled as a
square paperclip icon, but its drop and click zones were not constrained
to the visible square. They remained active across the whole 'Choose
File' button, which overlapped with the textarea. Instead, hide the file
input complete (display: none) and transmit click events from the
paperclip to the input programmatically.
Eventually, we'll need to address drag and drop events, but I want to do
that at the window level. Otherwise dropping a file outside the file
input drop zone causes the browser to navigate to the file://... url.
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.
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.
