gui, staking: Implement facilities for mandatory sidestakes and sidestake GUI #2781

Job	Run time
macOS 11 native [GOAL install] [GUI] [no depends]	10m 32s
Lint	1m 4s
ARM [GOAL install] [buster]	6m 3s
Win32	5m 31s
Win64	11m 11s
i386 Linux [GOAL install] [focal]	5m 2s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	24m 33s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	22m 4s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	19m 49s
	1h 45m 49s

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