Merge branch '877-save-invitation-outcome-to-invitation-message-and-make-available-to-ui' into 'master'

Store invitation outcome in metadata and make it available to the UI

This MR is based on !479 and should only be merged after that one has been merged as well.

It stores the invitation outcome in the message metadata  and includes it in the `canBeOpened()` calculation for private groups and sharables.

Closes #877

See merge request !480