This is a lightweight placeholder so calendar functionality can be
tested. Simply presents a message about the invitation, and a set of
yes/maybe/no buttons to click.
The UI is shown whenever the message appears to contain an invite.
There are many elements left to be done here:
TODO: response code (EAS protocol) doesn't seem to work
TODO: use real assets & design
TODO: provide a click-link into calendar event
TODO: show calendar icon in messagelist too
TODO: (if possible) persist user's response in button state?
