Follow #33127
This PR add backend logic and test for "anonymous access", it shares the
same logic as "everyone access", so not too much change.
By the way, split `SettingsPost` into small functions to make it easier
to make frontend-related changes in the future.
Next PR will add frontend support for "anonymous access"
Follow #30495
"HasAccess" behavior wasn't clear, to make it clear:
* Use a new name `HasAnyUnitAccess`, it will be easier to review related
code and permission problems.
* Separate everyone access mode to a separate field, then all calls to
HasAccess are reverted to old behavior before #30495.
* Add new tests.
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Co-authored-by: Giteabot <teabot@gitea.io>
