Add platform images only inside span.es_platform elts, use https:

[c12h]

Here's a draft fix for issue 114.

1. Make the 4 rules here that provide platform icons only apply inside elements.
2. Use https://... URLs for the platform images, avoiding 'mixed content' warnings

[c12h]

This is (1) the first time I've used github and (2) my first patch for Enhanced Steam, so mistakes are likely; please review carefully.

I've tested this in my Firefox 55 setup, and it worked.