Widescreen showcase image may not always appear on articles with feature as primary tone

Fixes issue when article has feature tone tag, but primary tone is something different, i.e.:

![screen shot 2014-11-05 at 17 30 59](https://cloud.githubusercontent.com/assets/813383/4922389/8dbf48ae-6511-11e4-9686-406ac2dbcfa2.png)

In this case, image goes full width, growing as wide as the browser window, which is incorrect.

This fix ensures the same logic is applied to the CSS as the Scala. Longer term, we may want to detach the logic around super-wide showcase images being so heavily attached to tones…