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/views/file_input_view.js

@@ -34,7 +34,7 @@ var Whisper = Whisper || {};
 
         addThumb: function(e) {
             this.thumb.src = e.target.result;
-            this.$el.append(this.thumb.render().el);
+            this.$el.find('.paperclip').append(this.thumb.render().el);
         },
 
         previewImages: function() {