Change NLU data format + migration to GraphQL

[znat]

Data format

• Data format should be compliant with Rasa 2.0 yaml format Example 1 Example 2

Database changes

• Every example should have its own database document
• canonical: should be a boolean under metadata

Export

• When exported, nlu files should be ordered alphbetically by intent names, and examples should be ordered alphabetically in every intent.
• An option to export in md or yaml should be available in export in rasa format

Migration

Write mongo migration (make sure the migration can still be run after being run (even if it worked fine))

Routing

• Filtered table accessible by url (not just app state) notably for Repurpose the NLU stats screen as the entry point of the NLU hub #711

[MatthieuJnon]

https://github.com/RasaHQ/rasa/blob/4c326f35bf3c67cef823752c6d3a370b08f43702/docs/docs/training-data-format.mdx