Implement 'Jump to first unread message' feature in rooms using m.fully_read marker

Seems at now RiotX miss Jump to first unread message feature, that scroll room timeline to first unread message. This is very important feature for most of users, especially in rooms with many members. Also X action for mark all as read.

I can’t find issue about implementing this feature, only about displaying read markers #84 in phase2. But displaying markers is not so important comparing to Jump to first unread message feature.

For implement jumping we can use m.fully_read marker from spec https://matrix.org/docs/spec/client_server/r0.4.0#fully-read-markers

So I suggest to implement Jump to first unread message earlier, than displaying read markers.

[jtagcat]

Note: some users prefer to jump to first, without losing their last read position (read few first unread, then jump back to their last position)

[bmarty]

Implemented by #592