Create model to store score and stats related to (entity, poll)

[amatissart]

Model Entity currently contains fields rating_n_contributors, rating_n_ratings, as well as tournesol_score introduced in #614.

These fields are actually specific to a given poll, and should be moved to a new model, with foreign keys to Entity and Poll. Potential name: EntityPollScore

[lfaucon]

My understanding of this issue is that it is not high priority because we only need this when we will have multiple Polls using the same entities. Please correct me if I understand this wrongly.

[GresilleSiffle]

• [x] create the new model
• [x] test the new model
• [x] create the data migration
  • see https://github.com/tournesol-app/tournesol/pull/1149
  • [x] update the comparisons view to create or refresh the model after each comparison
  • [x] update the ML train to also update the tournesol score of the new model
• update the serializers / update the API tests
  • is tracked by the EPIC https://github.com/tournesol-app/tournesol/issues/606
• update the Entity admin interfaces, and maybe others
  • is tracked by https://github.com/tournesol-app/tournesol/issues/1471

[lfaucon]

@bbosson can look at the PR #1149

[GresilleSiffle]

Before closing this task I need to create the remaining issues as described in: https://github.com/tournesol-app/tournesol/issues/631#issuecomment-1234372754