Default slots

[wochinge]

Proposed changes:

• add default slots upon domain creation to ensure fingerprints are created correctly (previously there was problem because we compared a domain without default slots with a domain which already contained the default slots - the test added to test_model visualizes this)
• deprecate add_categorical_slot_default_value, add_requested_slot, add_knowledge_base_slots. Their internal versions are automatically called when the domain is created
• don't persist the default value for categorical slots. We add it when the domain is loaded.

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[wochinge]

this was needed to work with the form which I added to the default domain.

[wochinge]

the presence of the RulePolicy in turn led to fallback, which is why I had to add the new story

[joejuzl]

Looks good! 🚀

[wochinge]

@joejuzl I suspect merge conflicts when merging 2.1.x to master. How about I handle the 2.1.x to master merge (will request your review) which you'd have done otherwise for the release?

[joejuzl]

@wochinge that would be great - thank you