Delete unnecessary faces from models, add cullface where it makes sense

[muzikbike]

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These shouldn't cause any visual differences outside of Spectator or otherwise clipping into blocks in unexpected ways, but cuts down on model file size a bit. The game also doesn't need to render as much, which is also a bonus.

Fixes #77, fixes #78 (for the former, cullface was left out for sign tops since it'd look weird with carpets on top, as well as several faces which the game refuses to cull since the block's hitbox doesn't extend out far enough, such as the thin sides of standing signs as well as chest locks)

[RaptaG]

This has been implemented in the latest version