Add "probation" mode for user accounts

[1011X]

Add a new "probation" mode for users, which would place them in a state where a certain number of reports from local users would automatically suspend their account. Only admins or moderators can put a user on probation, and only they can see if a user is on probation.

For example, if an admin or mod suspects an account to be a bad actor but no malicious activity can be pointed to as evidence, they can put the account on probation and, if any reports of malicious activity come in, the instance can automatically suspend them. This also works if someone want to give a user the benefit of the doubt, but would rather the user be suspended immediately on the first sign of toxic behavior.

Possible configuration options:

• Minimum number of reports: the minimum number of reports necessary before an account is suspended.
• Delay: how much time should pass before an account is actually suspended.
• Notification: whether mods and admins should be notified of automatic suspensions made by the server.

This feature could be used in conjunction with #73 or #51 to automatically put users who use suspicious account names or post suspicious content on probation. This would give a semi-automated moderation workflow.

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Questions to consider:

1. Do instance admins believe this feature would be useful?
2. Is there anything confusing about the feature or its options?
3. What options would you use or wouldn't use?
4. What would be some good defaults for the options you'd use?
5. Is there any way this feature could be abused?

[clarfonthey]

One immediate thing is that we have to somehow validate the reports coming in, as anyone from any instance can send them. That seems really easy to abuse imho.

Also, "probation" seems like a better term than "thin ice."

[1011X]

Perhaps I should've clarified that only reports from local users will count here. And "probation" does seem like a better term. I'll edit it.