Homepage refactoring and change ZA news on front page

This turns the homepage view into a class-based view to make it possible
to subclass the view to provide country-specific behaviour, rather than having
conditionals based on `COUNTRY_APP` in the core home page view.

It also refactors the presentation of news (recent blog posts) on the South
Africa front page, to reduce repetition.

This was all motivated by changing the news categories that were shown on the
front page of the South African site, and this also fixes #1350.

n.b. I changed the styling of the category headings, and it would be good to get
@wrightmartin or @zarino to see if they agree it's an improvement, or would do
it differently. I'll deploy this to my dev server to make that easier for one of them
to look at.