Change ApiGettingStartedGetPages to support 'morelike' suggestions

* Change to be a list/generator module instead of a standalone.
  This will allow us to easily use 'pageimages' and 'revisions'
  props (among others) via server-side piping.
* Factor out a PageSuggester interface.
* Implement CategoryPageSuggester for existing Redis functionality.
* Add MoreLikePageSuggester to use CirrusSearch to use 'morelike'
  functionality.
* Rename PageFilter to BasePageFilter, and add a subclass
  (CategoryPageFilter) to be used in conjunction with category-based
  filtering.

Bug: 67999
Change-Id: I90d8753efa87a4edc2d57cd67f64b8ec8d5a00ad
Merge "Change ApiGettingStartedGetPages to support 'morelike' suggestions"