Change to Font Awesome icon font (fixes #2138)

- remove Glyphicon assets and customize bootstrap CSS
- add Font Awesome v4.4.0 assets
- replace Glyphicons with Font Awesome icons in HTML
- add icons to modal headers
- add attribution for Font Awesome
- format HTML source code for buttons

gui/syncthing/core/syncthingController.js

@@ -69,9 +69,9 @@ angular.module('syncthing.core')
             'touch': 'Update'
         };
         $scope.needIcons = {
-            'rm': 'remove',
-            'rmdir': 'remove',
-            'sync': 'download',
+            'rm': 'trash-o',
+            'rmdir': 'trash-o',
+            'sync': 'arrow-circle-o-down',
             'touch': 'asterisk'
         };