feat: set the session locale from the Open edX preferred language

[pomegranited]

Description

If the full language locale is supported, use it in the session locale.
Otherwise, use the language part of the locale, if that is supported.

Falls back to "en" as the current default.

Closes openedx/openedx-aspects#298

Testing instructions

1. Login to the LMS.
2. From the Account settings, e.g http://apps.local.openedx.io:1997/account/ , update your preferred language to one of your configured `SUPERSET_SUPPORTED_LANGUAGES'.
3. Login to superset, e.g. http://superset.local.openedx.io:8088/login
   Note that the language used in the Superset UI matches your chosen preferred language.
   There may be missing translations -- Superset's translation coverage is not 100%.
4. From the Account settings, e.g http://apps.local.openedx.io:1997/account/ , update your preferred language to a language that is only partially-supported in Superset, e.g. Español (España) ('es-es').
5. Logout of Superset, and log back in.
   Note that your default language in Superset is now Spanish ('es').

Author Notes & Concerns

1. Unfortunately the solution proposed on Superset i18n: Save user's chosen language to user settings openedx-aspects#298 cannot easily be done, because Superset's User model is a FAB model, so adding fields is not straightforward. This was the next best solution I could come up with for Aspects.
2. Is falling back to the "language" part of a given locale always OK?
3. Should we also pass our django LANGUAGE_CODE through to the BABEL_DEFAULT_LOCALE (if it's one of the supported languages)?
   This would allow Aspects site operators to use a default other than "en".

[openedx-webhooks]

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