gui, staking: Implement facilities for mandatory sidestakes and sidestake GUI #2802

Job	Run time
Lint	1m 5s
ARM [GOAL install] [buster]	18m 19s
Win32	16m 45s
Win64	31m 30s
i386 Linux [GOAL install] [focal]	16m 15s
macOS 11 native [GOAL install] [GUI] [no depends]	22m 11s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	35m 22s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	24m 10s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	17m 58s
	3h 3m 35s

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