Improve safemode activation requirements

[CarsonBurke]

Things to consider when improving code:

• Should probably only safemode when a spawn is at risk.
• Spawns are probably not at risk if there is a sufficiently HP rampart on top of them
• Spawns can be killed in one hit by sufficiently fitted dismantle creeps, so checking if spawn hits are low isn't a good idea
• Perhaps check enemy range to spawn and spawn rampart existence for each spawn each tick

[CarsonBurke]

There is existing safemode code in defenceManager.ts. I suggest replacing that with this improved version :D

[ShuP1]

Partially implemented in ac67c3616b1b4ab8963833d5ed9b4c53ec13363d

Current implementation doesn't take ramparts into account, and only react to attack from previous tick (EventLog).

A spawn can be destroyed in a single tick. In this case, safeMode will trigger after spawn is gone.