This PR moves the logic from the setting-object mixin to a helper class. I've opted to maintain the interface of the previous classes (ThemeSettings / SiteSetting) that used the mixed-in methods through aliases so that we limit the amount of changes here (these are referenced in the string form and across classes/templates).
Another option we may consider in future if we want to optimize for performance is straight duplication which would trade off this overhead of aliasing/chaining calls through the helper for some duplicate code - only 2 models require these methods at the time of this PR.
As of #23867 this is now a real package, so updating the imports to
use the real package name, rather than relying on the alias. The
name change in the package name is because `I18n` is not a valid
name as NPM packages must be all lowercase.
This commit also introduces an eslint rule to prevent importing from
the old I18n path.
For themes/plugins, the old 'i18n' name remains functional.
