For performance reasons, we should only call `isHighDpi()` once on page load. @alexgibson suggested creating a general use high res object (maybe `Mozilla.HighRes`?) that contains functions for `convert_url_to_hires`, `init_hires_imgs`, and `isHighDpi`, as well as a local variable called something like `ishighres`. `isHighDpi` would only run the media query stuff the first time it was called, storing the result in `highres`. Subsequent calls would just return the value of `highres`. This object/library should probably live in `media/js/libs` with the other general purpose stuff.

During this refactor, would make sense to switch function names to camel case for consistency with other libs.