Fix significance model resolver, better tests, handling of upper case in query words

[glebashnik]

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Implements the following logic for resolving models:

• When language is explicitly tagged on query
  • Use language if available from the model registry, fail otherwise.
  • If “un” try both “un” and “en”.
• When language is implicitly detected
  • Use language if available from the model registry. Fallback to “un” then “en”, fail if none are available.

Handling of query words containing upper case chars.

[geirst]

👍 Just one comment to improve readability of the tests.