Unread badge/counts without push/notify

**Is your suggestion related to a problem? Please describe.**
Discord has the option to highlight a channel when new messages come in without causing a notification. This would be nice on Riot.

**Describe the solution you'd like**
Something like this:
with no audio or push notification.

**Describe alternatives you've considered**
Enabling notifications and dealing with a beep every time someone sends a message.

No files, tests, or entry points are named. Start by locating the Element client code for unread counts, channel highlighting, and notification preferences, then trace how audio and push notifications are triggered. Done means new messages can update an unread indicator without producing audio or push notifications, with the behavior covered by the relevant client tests.

Written by the indexing model from the issue text.