E2E: Key Backup Recovery Key setup UI does not allow backing out, is unclear when keys will actually be sent to the server

[Lrizika]

Description

When viewing the "Start using Key Backup" dialogue, clicking on "Set up with a Recovery Key" (under Advanced) brings up a series of dialogues that cannot be exited without backing up your keys to the server. It is also very unclear when keys are actually going to be sent - unlike in the primary dialogue, there is no warning.

Steps to reproduce

• Bring up Settings > Security and Privacy, click "Start using Key Backup"
  
• Under Advanced, click "Set up with a Recovery Key"
  -- From a UI point of view, this does not seem like an irrevocable (without exiting the application) decision
  
• A recovery key is generated. There is no longer any method to go back (or proceed without downloading or copying the key, see E2E: Key backup recovery key creation UI confusing #8878)
  
• After using one of the previous options, this dialogue is brought up. "Back" brings you to the prior step. "Ok" starts the key backup.
  
• Backup completed
  

This dialogue tree should be escapable. Moreover, there should be an explicit warning and confirmation when keys are about to be sent to the server - There is very little indication that the final "Ok" button will begin the process.

Version information

• Platform: web (in-browser)
• Browser: Firefox 60.7.2esr (64-bit)
• OS: CentOS 7.6
• URL: private instance (riot-web version: 1.2.2)