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). 
Hm, `links2` is kind of awkward. How about something like `/links-filtered/` as the base URL for these two that have a `filterRegex`?

Alternatively I do think this is an appropriate place for a query parameter instead of using the path. Having the `domain` in the path makes sense because when we are calling for `/links/` we need one, but a regex is an optional add-on. I could imagine having other parameters like limit, language, etc. and trying to cram those in the path sounds bad.

Right now I'm leaning toward this second option, what do yo think?