make_bugtracker_name creates names that are invalid

[Gavin Panella]

`c.l.database.bugtracker.make_bugtracker_name` could create a name containing an underscore.

If the URL is an email address and the local part contains an underscore, or if the URL is not an email address and the host part contains an underscore, `make_bugtracker_name` will return a name containing an underscore.

We should replace underscores with hyphens, and remove sequences of multiple hyphens (like `sed -r 's/-{2,}/-/g'`). Perhaps add an assertion using `IBugTracker['name'].validate`.