### Motivation
While using the browser default locale is often good enough, we probably
want more full control over locales, especially when we allow users to
choose their own locale — it's a poor experience to not have dates and
numbers formatted consistently with your chosen locale.
### Changes
Ensures (almost) all instances of `<sl-format-date>` as well as `Intl.*`
have the correct locale passed into them from our Lit localization
system.
The one exception here is in `frontend/src/utils/number.ts` where
ordinal suffixes aren't localized, so the locale is hardcoded to `en` —
I'll revisit this in the future.
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