[Outreachy Round 27] Add references and references_previous fields to Revisions table

[gabina]

What this PR does

This PR is part of the "Improve how Wiki Education Dashboard counts references added" project (read issue #5547).

The purpose of this PR is to add two new (integer) fields to the Revisions table:

• references: to store the number of references existing in a given revision id (AddReferencesToRevision class)
• references_previous: to store the number of references existing in the previous revision id (AddReferencesPreviousToRevision class)

This PR also provides a new regenerated erd.pdf file, and updates annotations on revision models and factories to reflect the changes.

Changes in app/models/wiki_content/revision.rb, spec/models/revision_spec.rb, and spec/factories/revisions.rb were automatically done by running annotate --models.

Open questions and concerns

• Notice that these new two fields will replace features and features_previous ones. We will need to remove them in the future (as well as all code related to them).
• db/schema.rb was automatically modified when running the migration locally, but manually modified after that, as some changes looked incorrect.

[ragesoss]

@gabina If possible, I would like to re-use the existing fields rather than adding these ones and removing the features and features_previous ones. Those features columns are serialized hashes, so they are flexible enough to hold the references counts for new data while co-existing with the features data from already-present revisions in the production database.

[gabina]

@gabina If possible, I would like to re-use the existing fields rather than adding these ones and removing the features and features_previous ones. Those features columns are serialized hashes, so they are flexible enough to hold the references counts for new data while co-existing with the features data from already-present revisions in the production database.

Yes, sure. In that case, I'm closing this PR as it won't be necessary.