When a blocked/opted out user tries to use BF, DM them an alert

[snarfed]

Hit a case recently where someone had #nobot in their profile bio, which BF interpreted as them opting out, but may not have been their intent. They tried to follow a BF-bridged account, and got confused when it didn't work.

When this happens, we could consider having BF DM them from a service account to say, hey, you're opted out right now so we're not doing anything, here's how to undo that if you didn't intend it and want to use the bridge after all.

[snarfed]

Heard from the user here: as expected, they hadn't intended #nobot to block bridges after all. Should we drop #nobot and only use nobridge to opt out?

[Tamschi]

As mentioned in #1466, it might be good to also inform accounts that fail the spam filter test of what the issue is.
Edit: Ah, I misunderstood the title change, so that was tracked here already 😅

[Tamschi]

Bridgy Fed should also Reject the Follow in these cases. It seems that leaving it as "pending" keeps causing some confusion.

[snarfed]

Inspired by #1519, we should also detect and handle when someone tries to DM prompt a spam-filtered user.