[Rails5] Permit concurrent loads

What does this MR do?

Rails 5 introduced a new (and safer) autoload mechanism which uses locking when loading missing constants. Under some circumstances it's possible that a deadlock occurs in multi-threaded environment (especially when running capybara tests)

To avoid these deadlocks, concurrent load is enabled when loading policies - we ere hitting timeout in this case: https://gitlab.com/gitlab-org/gitlab-ce/issues/48263

Note that autoloading is enabled only in development or test envs (not in production).

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
• Conform by the database 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)

What are the relevant issue numbers?

Closes #48263 (closed)