Add a sort button for subreddits in shortcut menus

When editing a subreddit shortcut, you can add multiple subreddits to create a dropdown menu. Currently, if you want the subreddits in the dropdown menu to be sorted, you have to do so manually. This pull request adds a button to sort the subreddits alphabetically if desired. It toggles between ascending and descending order if clicked repeatedly. You can see a screenshot of what it looks like here: http://imgur.com/8FFAOPC Constructive criticism or requests for changes are welcome!