This commit fixes a bug in the "Dark Mode" checkbox in the interface user
preferences where the checkbox state doesn't appear in the disabled
state if the user disables dark mode.
This happens because when rendering the checkbox, we check the relevant
user options field within the controller's `init` method, but at that
point in the controller's life cycle, the `user_option` object isn't
available. What we should do instead is move this check to the route's
`setupController` method where the `user_option` object is available and
we can set the correct state on the controller.
https://meta.discourse.org/t/-/349976 (private topic)
Why is this change required?
The flaky system test was due to the fact that we had to poll for the
user preferences interface page to reload after saving. However, this
turns out to be a bug on the user perferences interface page because the
page should only reload if the user has selected a new theme that is
different from the site's default but we were reloading the page for
users that did not have any user theme selected. Therefore there was an
unnecessary reload happening when saving other fields on the user
preferences interface page.
