move contexts from openbeta-graphql to sandbag

[musoke]

Comments in openbeta-graphql say that grade contexts should be defined in sandbag.
https://github.com/OpenBeta/openbeta-graphql/blob/dab9388193d4733e577c454a31354245253fdced/src/GradeUtils.ts#L5-L85

Move them over once current changes over there are settled and versions are synchronized.