Movie + InTheaters: Retina devices fix.

These fix does three things:
- is_retina is now deprecated so we're switching to DDG.is3x and DDG.is2x.
- Using is_retina fails on IE 10 and lower. Turns out undefined variables are invalid, but window.is_retina is valid.
- The posters are blurry on iPhone 6 Plus (3x screens) so we use the original image.
movie.js: update comment to add _org.
InTheaters + Movies: Revert change for a higher-quality image.
Movie + InTheaters: Update comment.
Merge pull request #1216 from duckduckgo/jag/movies-is-retina

Movie + InTheaters: Retina devices fix.