dashboard/app: make static resources work for dev_appserver.py #18486

Job	Run time
race	6m 57s
aux	4m 29s
build	4m 52s
gvisor	9m 14s
race_dashboard	3m 50s
dashboard	2m 5s
arch (presubmit_arch_freebsd)	6m 24s
arch (presubmit_arch_darwin)	3m 58s
arch (presubmit_arch_openbsd)	3m 45s
arch (presubmit_arch_netbsd)	3m 48s
old	7m 6s
arch (presubmit_arch_windows)	3m 35s
arch (presubmit_arch_executor)	3m 14s
arch (presubmit_arch_linux)	10m 44s
	1h 14m 1s

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