GuideMedaProgress: Allow adding duplicate URLs
Offline guides: Display live images if possible

Otherwise the thumbnail image won't be downloaded until that guide is
being processed.
Fix syncing guide with missing intro image

The problem was twofold:

1. We were trying to download an image at the URL ".standard" which
   failed.
1. Once we fixed it, the progress was 0/0 in the DB so the progress
   button crashed because 0 is an invalid max value.