Ability to hide and also unsubscribe from DMs

[taoeffect]

Problem

A user might want to "delete a DM".

Solution

Because consent is involved here, we can't actually delete the DM itself, but we can make it so that their client effectively removes it from the UI and forgets about that specific DM (unsubscribes from it too).

Note that closing this issue will also require thought as it relates to accounting, because these DMs are owned by someone, and we can't lose that reference of who owns them as otherwise it'll be garbage that will forever be stuck on the server with no way of removing it.