Adding missing indexes migrations from taggings table

API requests were timing out because tag_id was missing an index. These migrations were imported by running https://github.com/mbleigh/acts-as-taggable-on#post-installation:

bundle exec rake acts_as_taggable_on_engine:install:migrations

It looks like the acts-as-tagglable gem added the indexes in v4.0.0, but when we upgraded from v3.5.0 (back in 2016 via f571aeb5) we did not add them.

On staging, there are about 10.7 million rows on the taggings table. It took about 30 seconds for each index to be created.

On production, there are about 17.4 million rows, so I suspect the time to be about a minute per index.

Closes #43927 (closed)