Health bars now track green to red

But it's not great, maybe go to red sooner, and want to look more
yellow in the intermediate.  Kind of brown right now.  Probably
because low overall colors.  Really want:

green -> yellow -> red
0,255,0 -> 255,255,0 -> 255,0,0
Colors now green->yellow->red

New method get_life_color for units is pretty good

But sometimes on initialization, there's an overflow kind of error
with the red, and we get way too much...
Fix bug with health bars starting red

I was setting info before setting max_unit_hp, so the max_unit_hp
was some arbitrary value, usually 0 or 1, creating a wacky
life_fraction value and maxxing out the red with no green.  Change
order to set max_unit_hp first and it's all good
Add health bar background

Also switch to using rectangles for health bar.  Want to have an
outline for the bg, but it doesn't seem to scale right.
Health bars properly outlined.

NB: Don't try to outline things that I scale, it will scale sides
all funny.  Instead, change actual size of the shape.

Maybe can't deploy?  Hard to tell with chromebook.