Improve style of notifications

Summary:
  - Gets about 25% of the way toward @chad's notification mocks.
    - YES: Hover states, entire notification is a click target, border, header, footer.
    - NO: Profile pictures (lazy), timestamps (want to refactor time code before introducing a new formatting style), app icons (they'd look funny without timestamps I think)
  - Deletes some old files.
  - Mostly trying to get this good enough to turn on by default.

Test Plan: Looked at notifications. Clicked some notifications.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4119

src/applications/notification/controller/PhabricatorNotificationPanelController.php

@@ -26,8 +26,12 @@ final class PhabricatorNotificationPanelController
         '</div>';
     }
 
-    $content .=
-      '<div class="phabricator-notification view-all-notifications">'.
+    $content =
+      '<div class="phabricator-notification-header">'.
+        pht('Notifications').
+      '</div>'.
+      $content.
+      '<div class="phabricator-notification-view-all">'.
         phutil_render_tag(
           'a',
           array(