#3817715 Refactor code to manage Security Checkup items in dedicated components

In addition to the above, I pushed all the state/querying dependencies into the components themselves, as per @klimeryk‘s suggestion in this comment on #43101.

The largest change in this area was for Two Factor Authentication, where we’re now accessing user settings in a cleaner fashion instead of interacting directly with the passed-in UserSettings object.

I also added a check for Two Factor backup code verification that is only performed when 2FA is enabled.

Changes proposed in this Pull Request

• Move all the row-specific logic out of SecurityCheckupComponent, and push it into row-specific components
• Re-implement the 2FA row logic to use helper functions for the user settings
• Add a row/check for whether the user has verified their 2FA backup codes
• Simplified the email language as per @wensco‘s suggestion in #43101

Testing instructions

Using calypso.live with ?flags=security/security-checkup added after the build loads, or a local build of Calypso, check the following:

• Inspect the UI and make sure it loads and shows the 5 rows/options from #43101, with the “set to” removed from the account email item.
• If you have 2FA enabled, make sure that a row appears for “Two-Step Backup Codes”