Merge pull request #5910 from Goober5000/campaign_load_refactor #543

Job	Run time
Linux (Debug, gcc-5)	7m 23s
Linux (Debug, gcc-13)	8m 23s
Linux (Debug, clang-16)	10m 35s
Linux (Release, gcc-5)	5m 39s
Linux (Release, gcc-13)	7m 25s
Linux (Release, clang-16)	10m 4s
Linux (Debug, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	8m 55s
Linux (Release, gcc-13, -DFSO_BUILD_WITH_OPENGL=OFF -DFSO_BUILD_WITH_VULKAN=OFF)	7m 15s
	1h 5m 39s

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