dashboard/app: make static resources work for dev_appserver.py #18473

Job	Run time
dashboard	2m 32s
race	7m 20s
aux	3m 54s
race_dashboard	2m 54s
build	5m 11s
old	7m 12s
arch (presubmit_arch_openbsd)	3m 44s
arch (presubmit_arch_linux)	10m 43s
gvisor	4m 7s
arch (presubmit_arch_darwin)	3m 51s
arch (presubmit_arch_executor)	3m 16s
arch (presubmit_arch_windows)	3m 36s
arch (presubmit_arch_freebsd)	6m 20s
arch (presubmit_arch_netbsd)	3m 50s
	1h 8m 30s

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