Replace "article" with generic wording for content pages

There is one namespace selector "Articles" left. It could maybe
become "Content namespaces" or at least "Main namespace",
but I suspect that's better in a separate commit.

<https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Mobile-frontend-lead-image-tutorial-summary/en>

Change-Id: Id21c06a5b68648730ce163670827370e176cf5c7
Merge "Replace "article" with generic wording for content pages"