Implement "Mark as Unread from this message"

[nunoperalta]

Your use case

What would you like to do?

We should be able to "Mark as Unread from this message" inside a thread, like it's possible on Skype.

Why would you like to do it?

User stories can be found here: #347 (comment)

Example:

User story: I have a workflow where I "peek" what a user has sent me, to quickly read through and get any important/critical points, and if its non-critical, I mark unread until the point that I haven't "processed" yet.

Basically, I make sure that I'll encounter it later, in exactly the same spot that I found it, so that next time, I can go through it with a better swing.

Marking the entire channel as unread does not allow me to mark a specific point of "read from here", and so there needs to be extra non-obvious context clues pointing to where I had already read, especially if its a quick discussion or a long back-n-forth DM that I needed to read in-depth at a later point.

How would you like to achieve it?

By using unread markers, implemented by matrix-org/matrix-spec#1893