Gravity: Fixed resetting gravity after climbing a ladder

Ladders set the gravity to 0.0 and back to 1.0 when leaving the ladder.
Reapply our own value when a player leaves a ladder.

Thanks to DorCoMaNdO!
https://forums.alliedmods.net/showthread.php?t=240092
Gravity: Disable gravity, when upgrade gets disabled

Reset the gravity to 1.0 again, when the upgrade is disabled or the
plugin gets unloaded.
Gravity: Fixed not resetting when selling all levels

When a player sold all his gravity levels he would still be left as if
he bought level 1 until respawn.