* Create new namespace mw.popups.gateway to contain them
* Create folder resources/ext.popups/gateway/
* Split gateway{,.test}.js into gateway/{mediawiki,rest}{.test,}.js
* Extract stateless functions out of factory scope now that there are no
name collisions between the factories.
Bug: T123445
Change-Id: Ib256871c3e4cfe3f13361cb66d4e9a67e9823c7b
