gui, staking: Implement facilities for mandatory sidestakes and sidestake GUI #2785

Job	Run time
Lint	1m 11s
ARM [GOAL install] [buster]	3m 5s
Win32	4m 34s
Win64	9m 35s
macOS 11 native [GOAL install] [GUI] [no depends]	9m 51s
i386 Linux [GOAL install] [focal]	2m 51s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	25m 13s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	26m 55s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	16m 20s
	1h 39m 35s

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