Adds a sort button for subreddits in shortcut menus

When editing a subreddit shortcut, you can add multiple subreddits to create a dropdown menu. This adds a button to sort those subreddits alphabetically if desired. It also toggles between ASC and DESC if clicked repeatedly. See screenshot here: http://imgur.com/8FFAOPC
Merge pull request #872 from Timberline/master

Add a sort button for subreddits in shortcut menus