Fix localization for dependencies (fixes #1132)

This fixes localization for dependencies by explicitly importing locale
files in dependencies. I am not fond of the scattering it implies, but I
haven't found any cleaner solution:

- on the server, all the locales are imported. It seems fine to me,
  because we don't really care about the disk storage / memory impact on
  the server.
- if we imported the moment locales in the shared helpers, we'd need to
  import `moment/dist/locale/fr` (e.g.) because vitejs requires that,
  but then this is not processed by ts on the server, so the latter will
  crash at startup.
- instead, we have to put the moment locales in another file, that's
  only there in the client. Opted for `client/helpers/index`.
- flatpickr locales also have to be explicitly imported; did that in a
  new `client/components/ui/flatpickr` file.
- as a matter of fact, locale imports are now scattered in the client's
  code. Seems fine to me, as adding a new locale is not something we do
  that often.
1 job for fix-dep-l10n in 2 minutes and 11 seconds (queued for 8 seconds)
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passed ci #1459401