Ability to purchase stationary guns on a captured base buildings

Refactoring:
Removed stationary guns damage receiving
Removed mortar second operator

Bug Fixes:
Medic heal spot and engineer landmine place abilities buttons don't response in some cases
Units rotation doesn't correspond to move direction after sprint
Unexpected units teleporting after several SHIFT-selection usages
Units selection doesn't work after several SHIFT-selection usages

New Features:
Ability to purchase stationary guns on a captured base buildings