Support upcoming iOS 15 notification avatars

[ShadowJonathan]

CC @pixlwave (from #4662)

iOS 15 will have support for "notification avatars", allowing users to see who sent them a message;

macrumors article summarising it, under "new look for notifications" (I couldn't find the exact dev reference)

[pixlwave]

Thanks, I believe we need to implement #3407 as a start for this, but would need to double check.

[pixlwave]

Link to the WWDC talk with all the information about this: wwdc21-10091. The changes include the ability to show the room name separately to the sender name which would be nice.

We should confirm what happens to the notification priority once we start building against the iOS 15 SDK, as without opting into this, (or Time-sensitive Notifications if they're appropriate), there's a chance users could end up with notifications being rolled into the notification summary and not being delivered realtime.