Update the unique index on tags
Update the unique index on tags

Previously we required Tag#tag_id to be unique across all tags, but
actually it should be unique within the scope of a given tag_type.

alphagov/panopticon,theodi/panopticon,theodi/panopticon,alphagov/panopticon,theodi/panopticon,alphagov/panopticon,alphagov/panopticon,theodi/panopticon