database schema is defined redundantly with brain-score.web

the database schema is defined in 2 places:
1. in brain-score.web https://github.com/brain-score/brain-score.web/blob/master/benchmarks/models.py
2. and in brain-score https://github.com/brain-score/brain-score/blob/master/brainscore/submission/models.py

this makes it difficult to maintain and we should discuss how to unify them. Eg have brain-score.web import the models from brain-score