Fix server→client command list order (cpd wasn't being found).
Start redesigning jetpack to be a passive upgrade that integrates with regular movement control.

The current in-progress version comes without any weaknesses or limits such as
fuel. Use g_debugMove 1 to debug.
Improve Jetpack controls.
Don't apply sprint modifier in air.
Make jetpack use a refilling fuel.
Make jetpack fuel display pretty.
Disable jetpack for a short time when hit by another client.
Merge branch 'gameplay/jetpack', remote-tracking branch 'remotes/origin/master' into gameplay/testing