Add support for wiki farms

I have not tested the application on a wiki farm (multi-wiki setup), but based on how I wrote the database queries, I am pretty sure that it would pull dead links from every wiki on the XWiki instance, with no distinctions made among sub-wikis. This may even happen when the extension is installed on one sub-wiki. (I do not know!)

If someone could test this application out on a wiki farm and respond with the results, that would be super helpful toward informing the project's direction. Thanks!