Make it possible to mark reactions as "seen"

[iequidoo]

There is https://github.com/deltachat/deltachat-android/pull/3377 which adds notifications for reactions. But it's blocked because when the user receives a reaction notification on multiple devices and clicks "Mark read" on one device (or, equivalently, enters the chat), then the notification won't be removed from the other devices.
This should be fixed by remembering the rfc724_mid of reaction emails and marking them as seen on the server just as normal messages. Then, the second device can notice that they were marked as seen and send a MsgsNoticed event. I started implementing this in hoc/reaction-notifications-2, but got distracted (I didn't get very far, just half an hour or so, and I don't even know whether the approach I started with was the right one).

I've taken a look at this, but for me it seems simpler to add reactions as hidden=1 messages to the msgs table as well, then the existing mechanism of "marking seen" works for them. Also this allows to have Param::OverrideSenderDisplayname for reactions which may be useful for bridge bots, that was mentioned in the closed #5184. Currently reactions just go to the trash chat. Not sure if this is actually simpler because many changes to tests may be needed. CC @Hocuri @r10s @hpk42 @link2xt