Regex search

This patch adds the "Filter Links" menu option. The user can enter the exact domain, and a regex that will be matched against the full url for each link. If the regex matches, the link will be included in filtered results, otherwise it will not. The filtering is embedded in the url of the resource, so there is no additional limit needed -- each time you push `next` on the Links page, you get a new set of links, that are filtered against the previously posted regex. Protocol, domain, subdomain, and path searches can all be specified (since the text of the entire link is used as the target of the regex). 
I think we'll want a test here where a regex contains a `/`. I could imagine this being a problem with the controller, but not sure.