The Page Previews A/B test was disabled in T144490. After the changes
for that task had been deployed we discovered that the old definition
for when previews are enabled was invalid.
Previews are enabled:
* If the beta feature is enabled and the user has enabled the beta
feature.
* The user hasn't disabled previews via the settings modal.
* And soon, if the beta feature is disabled and the user hasn't disabled
previews via their user preferences (see T146889).
Changes:
* mw.popups#createExperiment -> mw.popups#isEnabled
* Make mw.popups#isEnabled act like a factory to maintain backwards
compatibility.
* Update the associated tests.
Bug: T152687
Change-Id: I713a90dfb29866b27738e7d19e8a22518a12d417
