Maintain max width / height info to speed up image downloading

If we ever download an image's original, we'll save its width and
height. This ensures that we don't try to fetch thumbnails that are
larger than the image itself too often, reducing 500s and improving
performance much.

TODO: Save image's width / height during upload process itself

GitHub: https://github.com/wikimedia/apps-android-commons/pull/8
Change-Id: Ice11cfabab96999b0d07b965d4515b47c1fff428
Merge "Maintain max width / height info to speed up image downloading"