Make the "Redirect URL" column sortable. 

Note: The ordering is not URL-aware. This means it sorts URLs just like any other piece of text. For example, https://zzz.com will be placed *before* http://aaa.com due to the "s" in "https".

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@1027208 b8457f37-d9ea-0310-8a92-e5e31aec5664