"Mark room as Unread" with a 'dot' on the room list

### Your use case

Users should be able to mark a room as unread from the context menu in the room list (long-press on mobile, 3 dots on web & desktop).

In order to keep this deliverable simple we will:
* Add a dot or other visual element to the room - there's no count or message specificity, read marker or read receipt movement. Just the introduction of a visual thing on the room list to jog the user's memory or remind them to 'come back later'

We will _not_:
* Make this a cross-platform feature. Doing so would require engineers from each platform and some sort of user level storage which we may choose to do in the future but we want a quick and easy implementation that we can start with to judge the impact and usefulness of an unread marker.

### Related platform issues
* The iOS issue for this: https://github.com/vector-im/element-ios/issues/7253