Deny repository disk access in development and test

What does this MR do?

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19363 and !19437 (merged).

We have a feature flag that makes it an exception to access Git repositories directly, instead of via Gitaly. With this change this feature flag becomes active in test and development but not in production.

There is still unmigrated code that accesses repositories directly; this code is now fenced with allow_disk_access blocks.

Via https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6119 I am ensuring that the 'deny disk access' feature remains disabled in gitlab-ee test and development. I expect there to be extra code that needs to be fenced in gitlab-ee; I will resolve that in a follow-up MR.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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