Add plural support to mwe-upwiz-campaign-media-count-desc

Per https://translatewiki.net/wiki/Thread:Translating_talk:MediaWiki/About_MediaWiki:Mwe-upwiz-campaign-media-count-desc/en

Change-Id: Ibd9a7e73521235573b65ae056b162c2bed42d702
Merge "Add plural support to mwe-upwiz-campaign-media-count-desc"