Respect Locale when uppercasing

I added the correct `Locale` as a parameter to all usages of `uppercase()`.

There are only two usages of `lowercase()` that don't need no `Locale`:
* when selecting the keys in the theme dropdown
* when checking the extensions of images
1 job for !176 with 295-i18n-uppercasing in 5 minutes and 7 seconds (queued for 4 seconds)
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