Don't use style attrs for theme images

[muffinresearch]

Relates to mozilla/addons#1660

To avoid needing unsafe-inline for style-src we need to remove that from themes where it's used.

This can be either to use a img element + some CSS

or

Use style props directly (not the style attr) via jquery.css() and use a data-attr for the img src.

Needs checking on details pages and any other places the themes are presented.

[muffinresearch]

Closing this - we're not going to be able to do it because there's tons of inline style attrs in libs and plugins. As a result I think we'll leave unsafe-inline for style-src for now.