WIP: refactor: Import contexts from sandbag

[musoke]

openbeta-graphql defines grade contexts used to map climbs to grade scales. The front end needs to do this too, so the existing functionality was copied to sandbag.

Reuse that instead of having parallel implementations.

todo

• [ ] tests
• [ ] check on scope of migration
• [ ] merge https://github.com/OpenBeta/sandbag/pull/134 and release new version
• [ ] bump required sandbag version here