dashboard/app: fix parsing bug and accept "gs://" prefixes #14908

Job	Run time
build	10m 11s
arch (presubmit_arch_linux)	10m 32s
dashboard	3m 43s
aux	4m 46s
arch (presubmit_arch_freebsd)	6m 36s
gvisor	4m 10s
arch (presubmit_arch_netbsd)	4m 19s
old	6m 36s
arch (presubmit_arch_openbsd)	4m 18s
race	8m 33s
arch (presubmit_arch_darwin)	4m 22s
race_dashboard	4m 31s
arch (presubmit_arch_windows)	3m 21s
arch (presubmit_arch_executor)	3m 18s
	1h 19m 16s

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