Removed Exit Platforms for Safe Cases

Updated our code to stop Fabric of Reality platforms from generating
under exits all the time. Now they generate only when the supporting
blocks are replaceable (in which case they're replaced by FoR) or when
they're not opaque solids or have tile entities (then FoR generates on
top). The exit search algorithm treats replaceable non-liquid blocks as
air, so an attempt is made to build against the ground and not on top of
replaceable blocks. The area around the door is cleared of blocks to
avoid killing the player with lingering replaceable blocks that might be
harmful, like poison ivy.