Update renewal banner i18n

What does this MR do?

I'll be working in this area and decided to backfill the test suite so it'd be more easily refactorable for the feature I'm working on. In the process I also added I18n suport.

At first glance one could argue this has duplicate language and could be more concise, but I found it difficult to follow the GitLab best practices for externalization (particularly "splitting sentences") in it's previous implementation.

I also felt like although the code was more concise before, it was fairly difficult to parse, and more difficult to extend.

• backfill license_helper_spec
• refactor license_helper

I've followed the best practices outlined here: https://docs.gitlab.com/ee/development/i18n/externalization.html#best-practices

This is part of: gitlab-org/growth/product#102 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team