Volatile activities
This issue is probably best discussed in combination with #251 and #715 (closed)
Volatile activities are activities that do not provide a permanent trace, but are provided as singletons for their activity type. Such an activity might be interesting in apps like AlekSIS-App-Matrix, where we could track the last activity of a user in a chat room. As chat rooms can be very active, we probably do not want to keep a record for every read marker or message from every user, but instead have one activity "Read Matrix room xyz" carrying the last date/time of the last read marker.
We should discuss whether we want to have the Matrix (or whatever app) manage a real Activity
object, or whether we want a mechanism for apps to provide volatile user activity information.