Supporting changes:
* Add mw.popups.registerChangeListener, which registers a change
listener that will only be called when the state in the store has
changed.
Change-Id: Ibe6934058327c7f02f7d8092e74a667a5a1c600a
Changes:
* Make grunt:qunit run all QUnit tests in those modules whose names
being with "ext.popups".
* Add ext.popups/index.js, which initialises the mw.popups namespace.
* Add ext.popups/userSettings.js, which contains the code that deals
with interacting with the User Agent's storage.
* Add ext.popups/experiment.js, which contains the code that that
decides whether or not the user is in the experiment condition.
* Add tests for both units, converting existing tests where appropriate.
* Remove the associated code from ext.popups.core/ext.popups.core.js.
* Finally, dispatch the BOOT action against the store in
ext.popups/boot.js.
Change-Id: I697207677304bd49c7cfe1d37bb0a4af7810f387
