Bug 1024440 - Rename and modify ImageResolution to handle video files

* ImageResolution becomes MediaResolution, with the regex expanded to include a set of web-friendly video file extensions
* image-resolution.js becomes media-resolution
* New test to sanity-check that pickMediaByResolution handles .mp4 
* Includes fix to sort directory listing before processing(!) - fixes a nasty error where files could get removed and omitted entirely from the application.zip