Prepare deployments table for bigint conversion

What does this MR do?

Prepare conversion of deployments.deployable_id to bigint because this references ci_builds.id.

Related to #329535 (closed)

Migration output

Up

== 20210525085158 InitializeConversionOfDeploymentsToBigint: migrating ========
-- table_exists?(:deployments)
   -> 0.0016s
-- column_exists?(:deployments, :id)
   -> 0.0018s
-- column_exists?(:deployments, :deployable_id)
   -> 0.0011s
-- columns(:deployments)
   -> 0.0012s
-- add_column(:deployments, "deployable_id_convert_to_bigint", :bigint, {:default=>nil})
   -> 0.0077s
== 20210525085158 InitializeConversionOfDeploymentsToBigint: migrated (0.0476s)

== 20210525085325 BackfillDeploymentsForBigintConversion: migrating ===========
-- table_exists?(:deployments)
   -> 0.0013s
-- column_exists?(:deployments, :id)
   -> 0.0013s
-- column_exists?(:deployments, :deployable_id)
   -> 0.0011s
-- column_exists?(:deployments, "deployable_id_convert_to_bigint")
   -> 0.0015s
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.
== 20210525085325 BackfillDeploymentsForBigintConversion: migrated (0.0686s) ==

Down

== 20210525085325 BackfillDeploymentsForBigintConversion: reverting ===========
-- execute("DELETE FROM batched_background_migrations WHERE job_class_name = 'CopyColumnUsingBackgroundMigrationJob' AND table_name = 'deployments' AND column_name = 'id' AND job_arguments = '[[\"deployable_id\"],[\"deployable_id_convert_to_bigint\"]]'")
   -> 0.0025s
== 20210525085325 BackfillDeploymentsForBigintConversion: reverted (0.0027s) ==

== 20210525085158 InitializeConversionOfDeploymentsToBigint: reverting ========
-- remove_column(:deployments, "deployable_id_convert_to_bigint")
   -> 0.0012s
== 20210525085158 InitializeConversionOfDeploymentsToBigint: reverted (0.0058s)

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