Adding an "Attach files" button to the message composition pane.

Changes include:
   * New markup for the button in compose.html
   * A hidden file input field in compose.html
   * Added reference to the file input field in filedrop
     initialization in compose.js
   * A feature test and a click event binding for
     the "Attach files" button in ui.js
   * New paperclip icon reference in fonts.css
   * New general hidden display classes in zephyr.css
   * New composition pane button classes in zephyr.css

Fixes to the "Attach files" button commit e673bda...

Changes include:
   * Fixed the feature test for (new XMLHttpRequest).upload so
     it works in Firefox.
   * Renamed .button to .message-control-button
   * Removed stray newlines

(imported from commit c1f0834b74fd7120ec27db64ec380ffb3fa34633)

zephyr/static/js/compose.js

@@ -532,6 +532,7 @@ $(function () {
     $("#new_message_content").autosize();
     $("#compose").filedrop({
         url: "json/upload_file",
+        fallback_id: "file_input",
         paramname: "file",
         maxfilesize: 25,
         data: {