Rework notification colors

This MR redoes the notification colors to be a little more understanding.

• UnreadCount = Total amount of unread messages in a room. These are marked with a small white bubble without any numbers.
• NotificationCount = Amount of messages which were supposed to send a notification in a room. These are marked with a grey bubble and with their number.
• HighlightCount = Amount of messages which were supposed to be noisy (i.e., do a ring). These are marked with a green bubble, their number + the amount of NotificationCount

For reference, the earlier behaviour:

• UnreadCount = show nothing, but still have text in bold
• NotificationCount = show background color (which is hard to see against) + their number.
• HighlightCount = show green color + their number + NotificationCount

BUG: 454896
Fixes #507 (closed)