Don't assume Participant on login_required view

We allow users without participant info to browse available trips.
However, the `participant_signup` property was assuming that all viewing
users have a corresponding Participant object. Correct this assumption.
Put booted participants at top of trip's waitlist

Previously, if a waitlist had 10 participants and the last trip slot was
replaced by a driver, that last signup would go to the bottom of the
queue of ten. Change this so that the participant is placed at the top
of the queue.
Refer to updated lottery times in help pages