Allow Contestant.next_suitable to take exclude option as ids
Discard upcoming races when adding contestants

all except the first.
Validate presence of contestant name
Postponing contestants when someone is not present
Show contestant times; can retire contestants

Retired can be toggled
Allow postponing of any upcoming heat
Improve contestant registration wording
Reset derby button

Also buttonify "Add contestants" and make a better contestant empty state.
Turn off autocomplete for contestant names
Fix XSS vulnerability
Align Previous Heat rows with Standings rows
Derby master can redo heats
Contestant generation for testing