We still have 2 different mechanisms in place, maybe even 3:
* We simplify the CSS selector when we know a popup type is
disabled, and it's impossible an anonymous user can enable it
at run-time.
* We create that "initiallyEnabled" map that allows anonymous
users to toggle the individual popup types at run-time.
* This map is also used to check if the footer link should be
shown.
* There is also a wgPopupsReferencePreviews global that acts as
a "kill switch". However, this is not a pure feature flag,
but incorporates the user setting for registered users. This
is currently partly redundant (checking
`mw.user.options.get( 'popupsreferencepreviews' )` does the
same) and can be removed later when the feature flag is not
needed any more.
The footer link currently acts odd because anonymous users are
unable to enable ReferencePreviews, but get the footer link.
This patch introduces a 3-state model:
* `true` acts as before.
* `false` means a popup type is disabled, but anonymous users
can enable it (i.e. this is the opt-out behavior for anonymous
users).
* `null` means a popup type is not available at run-time, for
nobody. Anonymous users can't do anything about this.
Registered users must leave the page and change a setting.
Bug: T277640
Change-Id: Id8d1396c09cf0f706034a66f9cd3c880a8b33df8
