List all classes available for elements

We need the ml classes ID to publish them through CreateClassification. But an ML developer cannot (and should not) know their ID on the different Arkindex instances.

So we need to:

1. list all the ML classes for an element's corpus
2. store them, indexed by slug on the worker instance
3. give an easy way to the developer to get the class ID from a slug
4. use a cache to avoid loading ML classes for the same corpus over & over