Support "Important conversations"

[nickvergessen]

• API feat(conversations): Add "Important conversations" which still notify… spreed#14738
• Overview 📲 Add "Important conversations" which still notify during DND spreed#14881
  • will contain the design discussion and result
• A setting for important exists at the moment already, but should follow the server side setting in the future
  • Whether it should still bypass phone DND is to be discussed in the overview one