Improve the handling of switching link preivews on and off.
First make sure that the global setting is tied into the room setting, previously it was a bit of a patchwork that worked more by luck than judgement. The two levels of global and room level are properly tied together in a hierarchy.
Add a message in the room when global notifcations re turned off. This has caused confusion in the past when people don't realise there are 2 levels.