Add option to block direct messages from people you don't follow

[Gargron]

Fix #5326

I heard some discussion about communicating to the end-user that the message could not be delivered when it's blocked like this, however there are no such indications when notifications are blocked using any other means in NotifyService, and they are generally impossible to implement consistently between local and remote accounts and therefore undesirable. Plus since a direct message is not generally directed as much as it tags one or more people who may or may not see it...

[koboldunderlord]

Looks like this is just a face-lift on the changes Paul and i made and doesn't address the criticism of the change - if a user initiates a direct message with someone they don't follow with the option on, there is no way to see responses from said person, unless I'm misreading your changes.

[nightpool]

@Gargron I think it makes a lot of sense from a UX and product perspective to a do a Reject { Note } for DMs that get filtered due to this option, which would bring us to parity (IMO) with twitter's feature in this area.

I understand if you think that's too much complexity though

[nightpool]

LGTM. Errors are probably not critical here

[inmysocks]

I think that it is important to point out that letting an abuser know that their message hasn't gotten through causes more abuse because then they will try and find another channel to use for harassment.