Add an index for tag_id for performance boost
Add an index for tag_id for performance boost

We used to have a (unique) index on tag_id, which was swapped in
favour of an index on tag_id, tag_type.

Unfortunately, this massively slowed registering of (publisher) content
and probably other things as well.

alphagov/govuk_content_models,dinuksha/Test,dinuksha/Test,theodi/govuk_content_models,theodi/govuk_content_models