Rationalise license codes

What does this MR do?

Makes license addon strings predictable to generate from the symbols in License::FEATURE_CODES

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See discussion from https://gitlab.com/gitlab-org/gitlab-ee/issues/2466#note_34253374 onwards for context

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Prerequisite of #2466 (closed)