Convert Authorized Fetch to a setting and add support for hybrid mode

What does this MR do?

This PR reworks Authorized Fetch in several ways:

1. The configuration file entry checkActivityPubGetSignature is deprecated and should be removed after upgrading.
2. A new admin setting for Authorized Fetch is added under "Security". Detailed labels explain the pros and cons of each setting.
3. A new user setting for Authorized Fetch is added under "Privacy". This allows the user to override the admin's choice, but on for their own account.
4. Implements a new hybrid setting for Authorized Fetch, where signatures are enforced except for "essential metadata" such as Actor objects and public keys. User profiles returned in this way are redacted to remove all personal data.

When migrating, the value of checkActivityPubGetSignature will be automatically imported into the system-wide admin setting. User accounts will default to "Use staff recommendation", which falls back to the system-wide default setting.

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