space-elevator: change item placements for better gameplay

space-elevator: change item placements for better gameplay

- Moved item_strength to weapon_vortex position
Placing the vortex at the highest most exposed point (especially one
only reachable by blaster jump) means that when a good sniper gets it,
they can camp pretty much anywhere and deny everyone else access to it.
It makes for slow gameplay and/or rage quitting and is generally
considered a mistake in Q3 mapping too.
Placing strength here means you'll generally need some support from
your team to take it successfully.

- Moved weapon_vortex to weapon_devastator position
While it's possible to snipe from or to this position, its low
elevation limits sniping because the line of fire to each base
is partially blocked by its floor. It's unlikely a camper can completely
deny others' access to it here.

- Moved weapon_devastator to item_strength position
Getting the strength here often came down to luck and spam, rather than
tactics or teamwork.  It works fine for grabbing the deva though.

- Added weapon_machinegun in each base, removed it from centre slimepit
This placement worked well on gasoline_v3r1, allows respawning players
enough range to damage someone sniping from mid.
Having it in the slime annoyed people, or they ignored it.