[POC] chore: migrate to Rasa 2.0.0.a1

[NetBKarine]

Description

Proof of concept (incomplete) migration to rasa 2.0.0 alpha 1 (major feature of it is the Rule Policy - see RasaHQ/rasa#6088)
Changes include:

• Migrating all assessment flow (the basic one) and QA logic to rules (and some special cases needed to overwrite generic rules)
• Removing mapping, fallback and form policies to use rule policy (if all the dialogue was written in rules, augmented memoization and TED could also be removed)
• Using the FallbackClassifier to replace the NLU aspect of the fallback policy. This included renaming our unsupported intent fallback intent to "nlu_intent"
• Changing "none" symptoms value because a hack in the rule policy causes problems with a "none" value.

Related issues

Related changes

Checklist

• PR title follows Conventional Commits guidelines
• All relevant PR sections are populated, irrelevant ones are removed