Store invitation outcome in metadata and make it available to the UI
requested to merge 877-save-invitation-outcome-to-invitation-message-and-make-available-to-ui into master
This MR is based on !479 (merged) 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 (closed)