System checks should verify db/schema.rb is writable

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In https://gitlab.com/gitlab-org/gitlab-ce/issues/40480 a user had an issue where the migration check output didn't show anything wrong.

We don't know what caused that, but later, we noticed that their db/schema.rb file was not writable, even though it is set as such by Omnibus.

We should add a system check to verify that's writable, because otherwise we could get into an unexpected state.