Validate that abbr is slug-safe

We currently have one which is `directgov_www.actonco2`. This is a problem
because Rails interprets the . as indicating a format, but it is a format it
doesn't support, so Rails returns 406 Not Acceptable.

The immediate fix is to change the abbr on that site, but this should prevent it happening again.

An alternative fix would be to tweak the Site#to_param to make abbr slug-safe,
but doing it this way means that the abbr is always the slug and vice versa.