Draft: Prepare indexes on taggings for bigint column conversions

What does this MR do?

This MR enqueues async creation for indexes that will be needed to convert taggings to bigint - !66625 (diffs).

This is done to prevent the actual column swap migration running for an excessive length of time.

Database migrations

Up

$ bundle exec rails db:migrate:up VERSION=20210830052547
== 20210830052547 PrepareIndexesForTaggingsBigintConversion2: migrating =======
-- index_exists?(:taggings, :id_convert_to_bigint, {:where=>"taggable_type = 'Project'", :name=>:tmp_index_taggings_on_id_bigint_where_taggable_type_project, :algorithm=>:concurrently})
   -> 0.0033s
-- add_index_options(:taggings, :id_convert_to_bigint, {:where=>"taggable_type = 'Project'", :name=>:tmp_index_taggings_on_id_bigint_where_taggable_type_project, :algorithm=>:concurrently})
   -> 0.0000s
== 20210830052547 PrepareIndexesForTaggingsBigintConversion2: migrated (0.0189s) 

Down

$ bundle rails db:migrate:down VERSION=20210830052547
== 20210830052547 PrepareIndexesForTaggingsBigintConversion2: reverting =======
== 20210830052547 PrepareIndexesForTaggingsBigintConversion2: reverted (0.0120s) 

Does this MR meet the acceptance criteria?

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Security

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Related to #334823 (closed)