Handicap is no longer required in order to edit a registration entry.
In general we can't assume that all of the entries in the timing
dict will have a valid Handicap (maybe they were loaded in from
a csv so skipped the registration window validation) and I might
want to edit some data before assigning a handicap.
If a handicap is specified, we do however check that it is valid.
The valid format was changed to be standard %H:%M:%S format as
verified through time.strptime, as that is what we will be using
to work with handicap times anyway.