Prepare taggings table for bigint conversion

What does this MR do?

Prepares migration of taggings table to bigint.

Related to #329536 (closed)

Migration output

Up

== 20210520133032 InitializeConversionOfTaggingsToBigint: migrating ===========
-- table_exists?(:taggings)
   -> 0.0019s
-- column_exists?(:taggings, :id)
   -> 0.0030s
-- column_exists?(:taggings, :id)
   -> 0.0011s
-- column_exists?(:taggings, :taggable_id)
   -> 0.0011s
-- columns(:taggings)
   -> 0.0013s
-- add_column(:taggings, "id_convert_to_bigint", :bigint, {:default=>0, :null=>false})
   -> 0.0148s
-- columns(:taggings)
   -> 0.0012s
-- add_column(:taggings, "taggable_id_convert_to_bigint", :bigint, {:default=>nil})
   -> 0.0012s
== 20210520133032 InitializeConversionOfTaggingsToBigint: migrated (0.0942s) ==

== 20210520133440 BackfillTaggingsForBigintConversion: migrating ==============
-- table_exists?(:taggings)
   -> 0.0014s
-- column_exists?(:taggings, :id)
   -> 0.0021s
-- column_exists?(:taggings, :id)
   -> 0.0011s
-- column_exists?(:taggings, "id_convert_to_bigint")
   -> 0.0010s
-- column_exists?(:taggings, :taggable_id)
   -> 0.0012s
-- column_exists?(:taggings, "taggable_id_convert_to_bigint")
   -> 0.0010s
unknown OID 28: failed to recognize type of 'relfrozenxid'. It will be treated as String.
unknown OID 1034: failed to recognize type of 'relacl'. It will be treated as String.
unknown OID 194: failed to recognize type of 'relpartbound'. It will be treated as String.
== 20210520133440 BackfillTaggingsForBigintConversion: migrated (0.0694s) =====

Down

== 20210520133440 BackfillTaggingsForBigintConversion: reverting ==============
-- execute("DELETE FROM batched_background_migrations WHERE job_class_name = 'CopyColumnUsingBackgroundMigrationJob' AND table_name = 'taggings' AND column_name = 'id' AND job_arguments = '[[\"id\",\"taggable_id\"],[\"id_convert_to_bigint\",\"taggable_id_convert_to_bigint\"]]'")
   -> 0.0048s
== 20210520133440 BackfillTaggingsForBigintConversion: reverted (0.0051s) =====

== 20210520133032 InitializeConversionOfTaggingsToBigint: reverting ===========
-- remove_column(:taggings, "id_convert_to_bigint")
   -> 0.0013s
-- remove_column(:taggings, "taggable_id_convert_to_bigint")
   -> 0.0008s
== 20210520133032 InitializeConversionOfTaggingsToBigint: reverted (0.0059s) ==

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