beta: restore commonswiki

A few days ago, I have reduced the number of wikis on beta (see bug
46104, gerrit https://gerrit.wikimedia.org/r/#/c/55889/ ).  commonswiki
has indeed been removed there but we do need it for testing purposes.

That prevented MobileFrontend users from login on the wiki (bug 46649).

Change-Id: I880b05c868dc461d88806c460db78a3366de6083