Optimize images instead of multiple resizing options

Unless the user selected 'Original' for image processing, scale images down to
2048x2048. I chose 2048 since it'll allow for full width retina images
on most themes. Those with the odd giant image themes can always switch
to full size images.

Simplified media processing:

- Avoid unnecessary CGImage <-> UIImage conversions
- WPMediaMetadataExtractor can be replaced by a much simpler
  `representation.metadata`
- Removed WPMediaSizing
- Deprecated media.orientation
- Moved media creation to a new MediaService

Note that iOS seems to be ignoring CompressionQuality, although I don't
know why yet.

FIxes #1685