Move pipeline creation on MR create to Sidekiq

Sean McGivernrequested to merge
move-mr-pipeline-creation-to-sidekiq into master

What does this MR do?

Doing this in the HTTP request means that if pipeline creation is slow, the whole request can time out - and stop the pipeline being created.

Part of #198694 (closed).

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

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