Add add-mirrors endpoint on mirrorbrain

[rgaudin]

With our current mirrbrain configuration, there's no easy way to know which mirror holds a copy of a certain file. We used to rely on ?mirrorlist endpoint because all our mirrors were able to serve all clients.

Now that it's not the case, ?mirrorlist only lists the mirrors that can serve the client calling. That means it excludes all the [region|country|as]-only which doesn't match the calling client.

There is no other endpoint providing this information. Information is in the DB though.

Because it's very useful to have access to this information (and because we tend to forget that ?mirrorlist is not reliable anymore and panic), we could create such an endpoint.

I didn't create this on mirrorbrain repo because the motive is purely ops-related but the simplest way to do it is probably to tweak the mod_mirrobrain to send it. Saves us the trouble of deployment and connecting to the DB (but it's in C).