Merge branch '661-implement-ux-for-creating-a-private-group' into 'master'

Implement UX for creating a private group

This MR allows the user to create a new private group and select contacts to be invited into the group.

There are currently 6 commits starting with some small refactoring for code reuse and making more functionality available in the backend. Each commit could be split up into a dedicated MR if desired.

![create-groups](/uploads/5229f102ee2611c05d5e9d1e2aac510d/create-groups.gif)

Closes #661

See merge request !353