Fixes #6820

Readds missing check, makes hoverpods their own mech type and moves them
to another folder.
Fixes #6822
Cleans up hovererpod move code, adds ion trails

No need to duplicate the mecha move code when we're only interested in
drifting. The hoverpod also starts drifting when it runs out of power,
now.
Merge pull request #6823 from mwerezak/hoverpod

Fixes #6820, #6822, adds ion trails to hoverpods.