Flashback Inform opt-out

[Tanookami]

I'd like to add on to this, requesting the ability to set a boolean flag on a flashback as "inform_ok" (default: true/it will @inform you). And/or have the game send you a direct pemit (if you don't want an inform) that X flashback was updated by Y person. Similarly, giving you a status update upon login that you have unchecked flashbacks, as with "you have new mail".

I've quickly discovered I'm OCD about checking informs and it easily distracts me from an actual RP scene if I get random informs, spam half my screen with the inform message, only for it to turn out to be "oh X name posted to Y flashback is all". In other words, it feels like I'm getting informed of RP updates for a "scene" I'm "in", which is extremely redundant to someone like me, who can generally keep track of my flashbacks and doesn't want to later delete a LOT of informs.

Originally posted by @dvoraen in https://github.com/Arx-Game/arxcode/issues/121#issuecomment-501834519