Add API that canonicalises grades

We should have a way to canonicalise minor differences in grade formatting such as

- Brazilian grades: "VI sup" -> "VIsup" (see #158)
- strip redundant whitespace
- capitalisation
  - YDS a/b/c/d should be lowercase
  - v2 -> V2
  - e2 -> E2
- PG-13 -> PG (or vice versa)

Ideally, this would only be a difference in how a grade is displayed. The original input should be saved.