Reducer changes:
* Break the mw.popups.reducers.rootReducer test into @@INIT tests
for each reducer as there's shouldn't be any need to test that a
framework works as documented.
Changes:
* Move the mw.popups.reducers#preview, #eventLogging, and #settings
reducers into their own files in resources/ext.popups/reducers/.
* Make the associated tests mirror the new layout.
* Move the initialization of the mw.popups.reducers namespace into its
own file.
* Remove the mw.popups.rootReducer property in favour of a simple
private factory function per the reducer change above.
Change-Id: I94229d9ef1985f3806eec44c2e8234a5bbddc94f
