Allow to block issues mass positioning [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Introduces a new feature flag :block_issue_repositioning which when turned on
blocks issues relative_position updates.

This is a temporary solution to deal with DB heavy stress when imports of projects with a large number of issues gets imported.

For more information check: gitlab-com/gl-infra/production#4321 (closed)

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