Allow dots in usernames to support Google and LDAP auth
To prevent nonsense, usernames are currently restricted to A-Z0-9. Now that
we have Google and LDAP auth, a couple of installs want to allow "." too
since they have schemes like "abraham.lincoln@mycompany.com" (see Tnnn). There
are no technical reasons not to do this, so I opened up the regexps a bit.
We could mostly open this up more but I figured I'd wait until someone asks
before allowing "ke$ha", etc., because I personally find such names
distasteful and offensive.