Merge branch '678-contact-list-refactoring' into 'master'

Refactor contact lists, their adapters and items

This was supposed to be a preparation for #678 to make the contacts lists cleaner and easier to re-use for different use-cases. Turns out #678 can't use this work, but it is probably nice to have anyway.

During this work, support for multiple identities has been removed from the various contact lists.

See merge request !363