uses valid BCP47/RFC5646 RequestLocaleInfo.language_tags in locale selectors

[cfm]

Status

Ready for review, but this branch depends on and includes commits pending merge in:

• refactors Source Interface for semantic HTML5/ARIA markup #6041

Description of Changes

Closes #6056 by:

1. refactoring i18n.map_locale_display_names() so that the values of the global LOCALES dictionary are i18n.RequestLocaleInfo instances rather than strings; and
2. updating securedrop/{journalist,source}_templates/locales.html accordingly to use the RequestLocaleInfo.display_name property in user-facing text and the RequestLocaleInfo.language_tag property as required for the a[lang,hreflang] attributes to validate.

Testing

Follow #6056's steps to reproduce:

1. on develop (or, pending refactors Source Interface for semantic HTML5/ARIA markup #6041, on its branch5986-semantic-source-interface)
   1. The locale selectors fail HTML validation on their a[lang,hreflang] attributes.
2. on this branch 6056-language-tag
   • The locale selectors pass HTML validation on their a[lang,hreflang] attributes.
   • The locale selectors are otherwise unchanged.

Deployment

No deployment considerations.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

[zenmonkeykstop]

Test plan passes, changes LGTM