Fixup conversation page styles and functionality

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.

js/conversation_panel.js

@@ -24,7 +24,7 @@
         conversation.fetch().then(function () {
             new Whisper.ConversationView({
                 model: conversation
-            }).render().$el.appendTo($('#conversation-container'));
+            }).render().$el.prependTo($('body'));
         });
     };