Re-work Categories

[bobbermac]

Instead of a page with links for each category (like tags), promote the category content to front-page-like status:

• Refactor the home-page template to be a general "headlines" page
• Move general, headlines-oriented functionality into discrete functions
• Convert home-page generating function to a category page of type "front-page"
• For each category, have a category page of the given category's type
• Update category links on the sites to point to these new headline pages for each category
• Support good user experience for new category pages:
  • Update the category page to use full-width page
  • Add a left column that lists all categories in the system as stacked pills nav menu
  • Make right column the main content area that's like the home page / headlines (but for just the selected category)