Can change instance to be private if federation is enabled

[wyattjsmith1]

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• [X] Is this only a single bug? Do not put multiple bugs in one issue.
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Issue Summary

An instance can be made private even if federation is enabled. This will cause an issue when the instance is restarted as lemmy will check to see if an instance is both private and federated.

Steps to Reproduce

1. As an admin, make a federated instance private. This should succeed without any issues/warnings.
2. Restart the instance.

Actual behavior: There is an error about the instance being both private and federated. The instance will not start. The only way I was able to resolve this is by modifying the database manually. Expected behavior: This state should never exist, and we should surface this condition to the user better.

Technical details

I am about to post a simple PR to return an error if this specific case is attempted, but a better option may be converting the private_instance and federation_enabled fields to a single enum as they are mutually exclusive.