Cross-wiki notifications on test wikis

As was teased in this week's tech news, cross-wiki notifications are now available as a BetaFeature on testwiki and test2wiki. Simply go to your preferences, enable "Enhanced notifications", trigger a notification for yourself on test2.wikipedia.org (e.g. log out and edit your talk page), and open up your notifications flyout!

The next steps are going to be populating the backend on all wikis, and then enabling it as a BetaFeature on more wikis (T124234).

Please try it out! If you find any bugs, please file a task in the newly-renamed Notifications Phabricator project.