`discourse-common` was created in the past to share logic between the
'wizard' app and the main 'discourse' app. Since then, the wizard has
been consolidated into the main app, so the separation of
`discourse-common` is no longer useful.
This commit moves `discourse-common/(lib|utils)/*` into
`discourse/lib/*`, adds shims for the imports, and updates existing
uses in core.
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.
