Add a configurable emoji for each circle

[juliushaertl]

From discussion in https://github.com/nextcloud/server/issues/42433

We should have a configurable emoji for each circle to help for identifying them in the UI.

• Needs to be stored in the DB
• Needs to be added to the public API https://github.com/nextcloud/server/issues/43683
• Collectives might want to use this instead of the own emoji
• User interfaces need to be adapted to show it in
  • Sharing UIs web+clients
  • Probably mentions in talk
  • Circles management in contacts