Probe unknown and ambiguous external bug trackers for their type

[Gavin Panella]

This depends on bug 51215, and develops it some more.

Bug 45386 is adding support for Savannah - though not status syncing - and it's highlighted a problem: it's difficult/impossible to disambiguate between a Savannah URL and a SourceForge URL. We also have an existing problem where http://acpi.sourceforge.net/dsdt/ has been auto-created as a bug tracker: it's not a bug tracker at all, but the URLs look like Mantis URLs.

It might be more effective to probe new bug trackers to guess their type. That way we can automatically disambiguate between Savannah and SourceForge. We can also confirm that new trackers are indeed real trackers. For those that are not, like http://acpi.sourceforge.net/dsdt/, we can set their type to, say, INVALID or NOT_A_BUGTRACKER. This could also be used in the UI to hide auto-created watches.

If the probing is reliable enough then we can also stop asking users what kind of bug tracker they are trying to create. Just add it as UNKNOWN, marked as unprobed, then the next checkwatches run can ascertain what type it is - i.e. checkwatches does outstanding probes before getting status updates.

We don't ask users when we auto-create bug trackers from comments, so why not skip that question all the time?