Feature-flag migration integrity check

[mjuchli-da]

In case we need to alter a past migrations, a feature flag should allow disabling the check during CI.

See previous work:
#1715 (review)

[mjuchli-da]

Just learned from @pawelstepien-da :

you can just update the hash if you need to tweak past migration with yarn migrations:update-lock
it doesn't block dev doing whatever they need, but makes it very clear for creator and reviewer of PR that migrations changed