Rename ParticipantClass to OldParticipant; #494

We know from the casing and context that it's a class. The key thing
here is that it's the old one.
Ensure two decimal places in Decimal; #494
Factor set_tip_to out of tip.json; #494

This is in preparation for implementing set_tip_to using the ORM. Having
set_tip_to available apart from tip.json is needed for the test suite,
so that we're able to easily set up tips using the same mechanism as the
application itself.

Signed-off-by: Chad Whitacre <chad@zetaweb.com>