Entity Subscriptions use Entity Kinds instead of Entity Types

@Wilduck this updates django entity subscriptions to use EntityKind filtering rather than using content types as the preferred method of filtering different kinds of entities.

There are two schema migrations in this PR. One of them adds the entity kind field, and the next one removes the entity type field in the Subscription model. This allows users to perform their own data migrations.

I updated the version and also added some notes in the docs about these changes. I also updated all docs to reference the use of entity kinds rather than content types