Leverage getEntityRecord when getting taxonomies from core-data

[danielbachhuber]

Re: taxonomies function in core-data/reducer.js:

Instead of adding a new reducer/selectors/actions, we could just leverage the new entities abstraction to automatically generate those. https://github.com/WordPress/gutenberg/blob/master/core-data/entities.js#L6

Currently, the entities only support fetching by id getEntityRecord but we should definitely introduce a findAll/findQuery as well getEntityRecords( state, kind, entity, query )

We should also add more tests in the process.

From #6761 (comment)