Improve language on codename generation page

* Standardize on "codename" (not "code")
* Improve flow and phrasing in the "Important:" section
Emphasize why you need the codename when returning
Clarify best practices for storing codenames

* Uses the language from
  https://github.com/freedomofpress/securedrop/issues/638#issuecomment-60285846
* Re-orders the paragraphs to explain *why* the codename is
  important first, and then explain how to store it safely.
Fix unit test, broken due to wording change
Merge pull request #793 from freedomofpress/clarify-source-codenames

Clarify source codenames