Add feature flag to disable db-writes on git fetch over https [RUN ALL RSPEC] [RUN AS-IF-FOSS]

Merged Bob Van Landuyt requested to merge bvl-disable-git-http-db-writes into master

What does this MR do?

This adds the disable_git_http_fetch_writes feature flag. It allows diabling some non-essential writes during git fetches hopefully improving the apdex of some of the git endpoints.

Related to gitlab-com/gl-infra/scalability#883 (moved)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

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