feat: Copy grade contexts from openbeta-graphql

[musoke]

https://github.com/OpenBeta/openbeta-graphql has grade contexts which define subsets of grades to be used in different regions. These need to be used in the front end (open-tacos) too, so move them to sandbag.

Contexts need to know about disciplines, so move those over too.

Todo:

• [ ] tests internally
• [ ] tests with draft changes to openbeta-graphql
• [ ] tests with draft changes to open-tacos
• [ ] check licence (openbeta-graphql is AGPL, sandbag is MIT)

Version copied from: https://github.com/OpenBeta/openbeta-graphql/commit/9b0b35b8bbd88795cac1535d96da142919fdcc7a