Fixes #450. Seemed like a simple fix but I ran into a bunch of problems. Here’s a summary of the changes:

 * WPImageSpan appeared to be storing the exact same data as the MediaFile class. So I simply added a MediaFile instance to WPImageSpan and removed all of the places in code where the WPImageSpan data was copied over to the MediaFile.
 * Changed saveMediaFile() in wpDB to also update the media file db row if it already exists.
 * Fixed some naming conventions.
 * Changed WPImageSpan thumbnail to be larger in the post editor. Looks nice!
 * If a WPImageSpan is in the post editor, it will have an edit icon drawn in the lower-right corner to clue in the user that they can tap it to edit the image’s options.
* Removed some duplicated code related to creating image thumbnails.