Fix up some DB migration issues with Rails 3

This should be a pretty uncontroversial backport from the Rails 3 spike...

* The public_body_tags table had already been removed
* The index has_tag_string_tags generated a name longer than the Postgres maximum of 63 characters. It was ignored in earlier Rails versions, see: https://rails.lighthouseapp.com/projects/8994/tickets/6187-postgresql-and-rails-303-migrations-fail-with-index-name-length-64-chars