improve Contacts activity (group creation, federation invitations, ...)

[mahibi]

The Contacts activity needs to be improved because:

• conversation creation workflow is not intuitive.
  • Handling should be aligned with iOS, see
    • https://github.com/nextcloud/talk-ios/pull/1572
    • https://github.com/nextcloud/talk-android/issues/1463
• current Implementation is quite buggy
• adding https://github.com/nextcloud/talk-android/issues/3713 only makes sense when the activity is improved beforehand

As the contacts activity can be worked on without much side effects to other parts of the app, it's a good candidate for first migration steps to coroutines and jetpack compose. Lets use:

• MVVM
• coroutines
• compose (--> also removes "flexible Adapter")
• use code from/ extract Code to nc common lib? (esp. regarding coroutines/compose)

Be aware that adding participants to a Conversation is currently also handled with the Contacts activity.

[jancborchardt]

It’s best to align with how other native apps on the platform do this. For example, to be on par with Signal, we would have to do 2 changes:

• If you tap on any person in the list, it will immediately start a 1on1 (or switch to the existing one) instead of going to multiselect.
• We need to add a very first entry "New group" which then shows the same list but you can multiselect.

This both makes it much quicker to start a 1on1, and much clearer when you are creating a group.