WebClient: add toast notifications #1923

Job	Run time
Test build flags	1m 42s
Test with PgSQL/MySQL/Cockroach	19m 21s
golangci-lint	2m 16s
Build Linux packages (amd64, ubuntu:18.04, latest, amd64)	2m 40s
Build Linux packages (aarch64, ubuntu18.04, latest, arm64)	15m 45s
Test and deploy (1.21, ubuntu-latest, true)	14m 1s
Build Linux packages (ppc64le, ubuntu18.04, latest, ppc64le)	15m 12s
Test and deploy (1.21, macos-latest, true)	22m 32s
Build Linux packages (armv7, ubuntu18.04, latest, arm7)	17m 24s
Test and deploy (1.21, windows-latest, false)	24m 7s
	2h 15m 0s

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