Move pod timer to a PCC "build", with varying timer based on the PCC level.
A higher level PCC can send supply pods more often than a low level PCC.
Starts at 24 hours for level 1, 12 hours for level 20, and 8 hours for
level 30, with a reasonably-smooth progression between them.

Added a subsidize_pod_cooldown RPC to allow immediate resending of a pod
for 2e.
Sort the plan list by level/extra_build_level numerically, should make it
easier for humans to find the level they want.
Got extra-build-level backwards