Disable DMs by default. Show different notification for DMs from unknown accounts

[db0]

Requirements

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• Did you check to see if this issue already exists?
• Is this only a feature request? Do not put multiple feature requests in one issue.
• Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.
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Is your proposal related to a problem?

The spam problem in the fediverse is increasing and it's only going to get worse as we become more popular. The lemmy DMs are the most vulnerable area atm as it's not easy to implement automated anti-spam tools on them since they're private. This is not only limited to spam even, as abuse and harassment can also happen via DMs.

Most other social media services also have similar problem which is why a very common approach that popular social media use is two-fold.

1. Do not display DMs from accounts one has not accepted.
2. DMs from accounts one has not interacted before have to be accepted through a special process, before being displayed. The notification for those DMs is also different from the normal notification (so the notifications can be separated).

I want to suggest something similar for lemmy and also some more enhancements on this formula

Describe the solution you'd like.

The anti-spam/harassment features I would like to have for DMs are:

1. Do not display DMs from accounts one has not accepted.
2. DMs from accounts one has not interacted before have to be accepted through a special process, before being displayed. The notification for those DMs is also different from the normal notification (so the notifications can be separated).
3. Allow each user to specify the minimum account age an account has to have before automatically accepting DMs. This can prevent DMs being lost from people with estabilished accounts whom one can have reasonable faith are not spam. The user still retains the possibility to require approval for all DMs by setting this value to -1 or accept all DMs by default by setting this to 0.
4. Yet Unaccepted DMs from accounts should automatically disappear if that account is banned.
5. integrate this with Add "trusted" instance list #5088 and/or Extend the "hesitated" instance functionality. #5087 if/when those are implemented, which can allow, say, someone to accept all DMs from trusted instances, but not others.

Describe alternatives you've considered.

There's no alternatives

Additional context

No response