Changes:
* Add the ext.popups.experiment module
* Add the mw.popups.experiment.isUserInCondition function, which
returns a promise that resolves with true if the user is in the
experimental condition, otherwise false
* If the experiment isn't configured, i.e. wgPopupsExperimentConfig is
null or undefined, then the user isn't entered into the experiment,
providing a kill switch
Bug: T132604
Change-Id: I49597494273e3862711a32e4951c8598e6c8bf59
