[Code health] Clean up flow of state in SubmissionService

[Gino Miceli]

• Remove select and deselect methods and submissionId state; change submission id via NavigationService instead.
• Use combineLatestAll to combine at active survey, selected LOI, current user and submissionId$ in NavigationService. Use distinctUntilChanged on the result of combineLatestAll to avoid hitting db multiple times.
• Emit LOADING when any of the above changes, then loadSubmission$ - use distinctUntilChanged on return value of stream.
• Remove unused code from MainPageComponent

@rfontanarosa

[jcqli]

Related to Async issues with RxJs

[Gino Miceli]

Trying to make Beta scope tractable; let's tackle this as part of the push for GA.