Article template refactor

Huge apologies for the big pull request, this is mainly a refactor of the article template, and all it's coupled fragments. @mattosborn is taking on tone-based templates (not in this PR), and I was tinkering with showcase main media (present in this PR), so an improved article structure puts us in good shape to deliver these.

PR contains:
- Renaming and splitting article classes into classes called 'content'. The remaining classes will be done later
- Remove redundant classes
- Update some js-based classes
- Visual Tone server-side refactor
- Changes to article-like styles and templates to reach parity 
- Showcase main media for feature tone articles will be full-width