Refactor paranoid_verification

[olbrich]

We want to perform a general cleanup of many of our modules before you move to 1.0.

While there may be more issues with this particular module, a good starting point is to check the following:

• Are there any Rubocop violations we can fix?
• Are there practices we use elsewhere, but not in this module (e.g., ActiveSupport::Concern)?
• Can we improve the documentation (especially adding Yardocs)?

This can be taken in smaller pieces (e.g., one PR for documentation, another to use ActiveSupport::Concern, etc.) if it's easier to manage.