Replace i18next with something more suitable

[sluukkonen]

i18next is quite heavyweight and not particularly suitable for our use-cases. I think we could come up with a homegrown solution that would be

1. More type safe
2. More lightweight
3. More suitable for our use-cases

Just a simple TypeScript object like

const translations: Translations {
  someTranslationKey: () => 'foo',
  otherTranslationKey: (bar: string) => 'foo' + bar
}

and an accompanying hook

const t = useTranslation() // Retrieves the proper Translation-object for the current language
const whatever = t.someTranslationKey() 
// Alternate API
const whatever = t('someTranslationKey') // take a `keyof Translations` as an argument

might be an improvement.