[MIRROR] Adds `UPSIDE_DOWN` movetype for negative gravity / makes Atr… #1897

Job	Run time
Run Linters	2m 20s
Collect data for other tasks	13s
Compile Maps	4m 9s
Windows Build	2m 37s
Integration Tests (birdshot) / run_integration_tests	11m 10s
Integration Tests (blueshift) / run_integration_tests	11m 28s
Integration Tests (deltastation) / run_integration_tests	11m 31s
Integration Tests (gateway_test) / run_integration_tests	12m 37s
Integration Tests (icebox) / run_integration_tests	11m 33s
Integration Tests (kilostation2) / run_integration_tests	10m 51s
Integration Tests (multiz_debug) / run_integration_tests	10m 23s
Integration Tests (metastation) / run_integration_tests	10m 53s
Integration Tests (northstar) / run_integration_tests	11m 47s
Integration Tests (nssjourney) / run_integration_tests	10m 33s
Integration Tests (runtimestation) / run_integration_tests	10m 0s
Integration Tests (tramstation) / run_integration_tests	11m 45s
Integration Tests (voidraptor) / run_integration_tests	11m 5s
Alternate Tests (515, 1603, runtimestation) / run_integration_tests	9m 29s
Check Alternate Tests	1s
Compare Screenshot Tests	43s
	2h 45m 8s

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