gui, staking: Implement facilities for mandatory sidestakes and sidestake GUI #2788

Job	Run time
Lint	1m 5s
macOS 11 native [GOAL install] [GUI] [no depends]	7m 53s
ARM [GOAL install] [buster]	3m 55s
Win32	4m 53s
Win64	7m 2s
i386 Linux [GOAL install] [focal]	3m 33s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	9m 2s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	9m 4s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	9m 3s
	55m 30s

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