Fix up some DB migration issues with Rails 3

* 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
Merge remote-tracking branch 'openaustralia_github/db_migration' into develop
Drop the obsolete indexes explicitly by name - the 'remove_index' calls don't work as the table has been renamed, and they automatically generate the index name from the table name. Apply custom index name from a5e90e2b7ea6adedbf80c05bbb84425a02c63da0.
Merge branch 'feature/db-migration-rails-3-prep' into develop