Match Tor Browser and (new) Orfox UAS together

The two user agent strings are now nearly identical, we should try
combining the patterns (again). There are also new unit test strings
for this.

This roughly combines the pattern matching introduced in
749fd988 and
644e1c21.

We can remove the old Orfox-specific pattern (Gecko/38) after
users have a few weeks for upgrading to the new version.