WebClient: use flatpickr as time picker #1909

Job	Run time
golangci-lint	2m 25s
Test build flags	1m 41s
Build Linux packages (amd64, ubuntu:18.04, latest, amd64)	2m 22s
Test and deploy (1.21, ubuntu-latest, true)	14m 0s
Build Linux packages (aarch64, ubuntu18.04, latest, arm64)	15m 1s
Test and deploy (1.21, macos-latest, true)	18m 30s
Build Linux packages (ppc64le, ubuntu18.04, latest, ppc64le)	15m 0s
Test with PgSQL/MySQL/Cockroach	19m 12s
Test and deploy (1.21, windows-latest, false)	24m 12s
Build Linux packages (armv7, ubuntu18.04, latest, arm7)	17m 15s
	2h 9m 38s

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