Fix handling mailto: links in the contact group view

This relied on the string-based signal connection magic in
ContactDefaultActions, which was recently removed under the
assumption of it being unused.

Instead, don't do any mailto: link parsing (just for it to be
reassembled on the handling side anyway), but forward the clicked
URL unconditionally. That will still work with both ContactDefaultActions
now and in a near future where it is entirely gone.