dashboard/app: add targeting controls to the coverage page #14983

Job	Run time
build	13m 37s
arch (presubmit_arch_linux)	11m 10s
arch (presubmit_arch_freebsd)	6m 32s
aux	4m 49s
arch (presubmit_arch_netbsd)	4m 26s
dashboard	2m 44s
arch (presubmit_arch_openbsd)	4m 26s
arch (presubmit_arch_darwin)	4m 22s
gvisor	4m 14s
old	6m 36s
race	9m 9s
arch (presubmit_arch_windows)	3m 29s
arch (presubmit_arch_executor)	3m 15s
race_dashboard	6m 23s
	1h 25m 12s

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