Add API that canonicalises grades

[musoke]

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.