Story 1465: Add a call to action on main pages

Introduce MobilePage abstraction to help with continued use
of PageImages.

To test:
* Add wgMFKeepGoing = true to LocalSettings
* Edit MediaWiki:Mobilefrontend-keepgoing-wikify-category
to contain text:
Category:All articles with too few wikilinks
* Ensure at least one page is in the category
Category:All articles with too few wikilinks

Change-Id: I78ca8c2b93f0612d2a4f29e69ec0b61cda80a2bb
Merge "Story 1465: Add a call to action on main pages"