add block comment about user context for each VC

[tomholub]

Got it. Are these flows shown correctly?

• SetupInitialViewController -> SetupBackupsViewController (where I can also enter pass phrase) -> inbox
• SetupInitialViewController -> SetupManuallyImportKeyViewController -> SetupEnterPassPhraseViewController -> inbox
• SetupInitialViewController -> SetupGenerateKeyViewController (where I can also enter pass phrase) -> inbox

That means SetupEnterPassPhraseViewController is only used for importing keys. Is that correct? If yes, then let's name it in a way that suggests that they are related: SetupManuallyEnterPassPhraseViewController

Also, in another PR (I'm filing an issue), it would be good for each view controller to have a block comment above it where it's explained what is the view for and how user gets there and where user typically goes next. For example:

/**
 * user is sent here from SetupInitialViewController when some backups were found in his inbox
 * here the user can import their pass phrase that matches these backups to proceed with setup
 * after setup finalization, the user will be sent to `InboxViewController` with a list of emails
 */
class SetupBackupsViewController { ...

Originally posted by @tomholub in #323 (comment)