Introduce ability to bypass push mirror delay

What does this MR do and why?

For the operation of gitlab.com we rely heavily on push mirrors to https://ops.gitlab.net. We usually also mirror pushes of unprotected branches, as we run MR pipelines on ops as well.

The 5 minute delay incurred between pushes really slows down the feedback cycle and limits our ability to quickly mitigate incidents. In many cases it is the limiting factor when waiting for a merged MR to get applied.

This patch introduces a way to selectively bypass this delay on a per-project basis via ops feature flag.

It's a middle ground between what we have today, and a full-fledged configurable delay time that has been proposed in the past: #99 (closed), #3390, #3832 (closed).

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