Feat/killswitches

[kanoka42]

This PR addresses issue #943.

What this PR does

• Creates a new module, service, controller, and guard.
• Implements the guard in certain routes to programmatically switch off or on certain endpoints.
• Adds an endpoint for admins only that lets them update the current killswitches as well as retrieve which killswitches are currently active.
• Implements the logic in certain deep functions for finer control of killswitch targeting (new user signups)
• Updates unit tests to ensure the functionality of the killswitches

Closes #943

[kanoka42]

Closes #943

[tsa96]

I'm sorry, I mentioned adding a couple unit tests in users.service.spec.ts a couple comments ago then forgot to bring it up in my last review. Since we're already testing the findOrCreateUser method we should definitely be including a check for the killswitch guard in there.

Besides that everything looks good, will approve after those tests