Allow disabling private messages. Fixes #3640

[dessalines]

This adds a local user setting: enable_private_messages default true.

It then checks when sending to a local user, that both you and the recipient have that setting enabled.

It also checks on an apub private message receive, that you have it enabled, before creating it.

[Nutomic]

This is not enough, you also need to federate the setting as part of Person. That way pm buttons can properly be disabled for remote users too. I didnt find any existing property for this purpose, so you can simply call it enable_private_messages and add it to the context.

[richardARPANET]

This is greatly needed 👍 Can there also be a global setting?

[dessalines]

Global setting should probably be in a different PR / issue. This is specifically to prevent harassment on a user-level.

[Nutomic]

Outdated, feel free to reopen when you work on it again.