Prepare for generating school codes

[chrisroos]

This is mostly a refactoring PR in preparation for creating a school in Profile API upon verification in Editor API. Once we're creating schools in Profile API we'll need to keep them in sync with the data in Editor API. To make that easier this PR restricts a verified school from being rejected so that we don't have to handle updating/deleting schools in Profile API (at least for now).

This PR also:

• tightens up validation so that a school can only have one of verified_at or rejected_at set at a time, and so that once set those values can't be changed.
• adds tests around verification and rejection of schools in the admin UI before then simplifying those controller actions.
• fixes a bug in SchoolVerificationService#verify whereby a failed verification would appear to have been successful to the user.

[chrisroos]

Thanks @floehopper. Would you mind reviewing/approving it again now that I've addressed your comments, please? I've made the following changes since your last review:

• Used the admin_schools_path helper method in the commit titled "Add tests for Admin::SchoolsController"
• Removed the need to disable a RuboCop rule in a new commit titled "Avoid update_attribute in SchoolVerificationService spec"
• Updated the message of commit titled "Extract School#reject" to explain why it's not a bang method.