content ratings on detail page to new mocks/icons (bug 931947)

(Aiming for 12/17)

[Mock](https://bug931947.bugzilla.mozilla.org/attachment.cgi?id=8344778)

[New icons](https://www.globalratings.com/download_library-mozilla.aspx)

[ESRB Storefront Guidelines](https://bug928587.bugzilla.mozilla.org/attachment.cgi?id=820731)

**Notes**:

- rightfully switch from ```background-imgs``` to ```img```, with mapping from slugs to img srcs in ```settings.js```
- content descriptors in text next to content rating icon 
- new PEGI/Generic icons, use CLASSIND icons rather than building our own via CSS
- scale icons down to 48px width
- PEGI shows descriptors icons on the right, other bodies show interactive elements (PEGI's interactive elements are in the form of descriptors)
- less hard-coding PEGI stuff