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.
Why this change?
Previously, we were preloading the necessary metadata for
`adminCustomizeThemes.show.schema` route in the
`adminCustomizeThemes.show` route. This is wasteful because we're
loading data upfront when the objects setting editor may not be used.
This change also lays the ground work for a future commit where we need
to be shipping down additional metadata which may further add to the
payload.
Why this change?
On the `/admin/customize/themes/<:id>` route, we allow admins to edit
all settings via a settings editor. Prior to this change, trying to edit
and save a typed objects theme settings will result in an error on the
server.
