Add rule to control whether units can automatically rebuild destroyed blocks

[phinner]

This rule is quite useful. For players that want extra difficulty or specific Mindustry servers like lobbies or in my case a schematic showcase server (Clearing build areas without units rebuilding it).

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[Anuken]

The name is very misleading, it implies that it controls unit AI when the reality is that it controls whether "ghost" blocks appear at all.

[phinner]

The name is very misleading, it implies that it controls unit AI

I see. Hmm, now that I think about it, there is also the build tower affected by this.

What about renaming to rebuildDestroyedBlocks. It has the advantage to not specifically include units thus not directly implying that their AI is controlled.

Honestly, I don't have other ideas apart that.

[Anuken]

Renamed to "ghostBlocks", see commit.

[phinner]

Many thanks