Download higher quality images

[pserwylo]

Images currently download from Wikimedia Commons as a 1024px wide version. This was to ensure that the initial books provided with the app did not add too much to the filesize of the app itself.

However, for custom books, we want people to have the best experience possible, and so they should be able to get access to higher quality images. This should not make the editing process any slower though, so we should initially just download a low res thumbnail while first adding the page, and then in the background we should fetch a higher quality version of the image to save to disk.