Frontend/i18n: More locales constants

[tristanlabelle]

Introduces DEFAULT_LOCALE to replace "en" everywhere and disambiguate it from ALWAYS_AVAILABLE_LOCALES that prepares for #9184 , which will add a second always available English locale. (sometimes we want to test if it's the default locale, sometimes if it's an always available locale).

Also simplifies LANGUAGE_MAP to LOCALE_AUTONYMS since we only have one mapped property now, and adds LOCALE_COOKIE_NAME to codify "NEXT_LOCALE".

No functional changes. There are many files but most simply replace locale ?? "en" by locale ?? DEFAULT_LOCALE.

Testing

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Web frontend checklist

• There are no console warnings when running the app
• Added tests where relevant
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

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Backend

Schema	Schema diff	Sample emails	SQL query log no change in DB access patterns

Other

Protos