Ref #6463, #6428
* pin isort to 5.6.4
* update isort version mentioned in python_guidelines.rst
* make lint commands consistent between make lint and circleci
* fix isort errors on files outside of /wagtail
* speed up isort by ignoring additional directories and filetypes
* Fetch new translations from Transifex, update contributors, add new languages with >90% coverage to WAGTAILADMIN_PROVIDED_LANGUAGES
* Update transifex config to download Chinese (Simplified) and Chinese (Traditional) translations as zh_Hans and zh_Hant respectively
* Fix get-translator-credits.py to recognise 'tet' as the locale string for Tetum (which Babel doesn't)