Initialize conversion of ci_sources_pipelines.source_job_id to bigint

What does this MR do?

This MR starts conversion of ci_sources_pipelines.source_job_id to bigint.

See #328435 (closed).

Database migrations

Up

$ bundle exec rails db:migrate:up VERSION=20210422022825
== 20210422022825 InitializeConversionOfCiSourcesPipelinesSourceJobIdToBigint: migrating 
-- table_exists?(:ci_sources_pipelines)
   -> 0.0017s
-- column_exists?(:ci_sources_pipelines, :id)
   -> 0.0012s
-- column_exists?(:ci_sources_pipelines, :source_job_id)
   -> 0.0010s
-- columns(:ci_sources_pipelines)
   -> 0.0012s
-- add_column(:ci_sources_pipelines, "source_job_id_convert_to_bigint", :bigint, {:default=>nil})
   -> 0.0021s
== 20210422022825 InitializeConversionOfCiSourcesPipelinesSourceJobIdToBigint: migrated (0.0311s) 


$ bundle exec rails db:migrate:up VERSION=20210422023046
== 20210422023046 BackfillCiSourcesPipelinesSourceJobIdForBigintConversion: migrating 
-- table_exists?(:ci_sources_pipelines)
   -> 0.0016s
-- column_exists?(:ci_sources_pipelines, :id)
   -> 0.0011s
-- column_exists?(:ci_sources_pipelines, :source_job_id)
   -> 0.0009s
-- column_exists?(:ci_sources_pipelines, "source_job_id_convert_to_bigint")
   -> 0.0009s
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.
== 20210422023046 BackfillCiSourcesPipelinesSourceJobIdForBigintConversion: migrated (0.2749s)

Down

$ bundle exec rails db:migrate:down VERSION=20210422023046
== 20210422023046 BackfillCiSourcesPipelinesSourceJobIdForBigintConversion: reverting 
== 20210422023046 BackfillCiSourcesPipelinesSourceJobIdForBigintConversion: reverted (0.0089s)


$ bundle exec rails db:migrate:down VERSION=20210422022825
== 20210422022825 InitializeConversionOfCiSourcesPipelinesSourceJobIdToBigint: reverting 
-- remove_column(:ci_sources_pipelines, :source_job_id_convert_to_bigint)
   -> 0.0015s
== 20210422022825 InitializeConversionOfCiSourcesPipelinesSourceJobIdToBigint: reverted (0.0057s)

Does this MR meet the acceptance criteria?

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Related to #328435 (closed)