gui, staking: Implement facilities for mandatory sidestakes and sidestake GUI #2804

Job	Run time
macOS 11 native [GOAL install] [GUI] [no depends]	8m 33s
Lint	1m 1s
ARM [GOAL install] [buster]	3m 41s
Win32	4m 31s
Win64	7m 14s
i386 Linux [GOAL install] [focal]	3m 6s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	26m 54s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	28m 58s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	18m 19s
	1h 42m 17s

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