Updating README with TODOs.
Fixing some failing tests after @aedff9.
Adding checkin status to tickets.

This consists of a boolean field in the tickets table, where the value
indicates whether or not the ticket has been used to check in at the
event. A toggle/checkin button is added in the participants interface.
Adding logging of checkins and checkouts.
Removing a finished TODO from the README.
Only show participants with confirmed payments.

The interface for showing each participant to an event will now only
show users with tickets with confirmed payments, compared to a tab view
in which users with unconfirmed or expired transactions would be shown.
Bug in html table markup.