Refactor feed refresh logic and feature test css-generatedcontent
Refactor feed refresh logic and feature test css-generatedcontent

* Rename krRTRC_Refresh to updateFeed.
* Rename krRTRC_hardRefresh to updateFeedNow.
* Remove redundant var rcRefreshEnabled.
* Move rcPrevDayHeading resetting to where it is used instead
  of afterwards from pushFeedContent.
* Add simplified version of Modernizr to feature test
  generatedcontent.

Krinkle/mw-gadget-rtrc,he7d3r/mw-gadget-rtrc