Refresh cached cover art

Previously, cached podcast cover art was never invalidated. When podcasts updated cover art, it would not be updated locally.

To correct this, time-based and hash-based caching have been introduced. The time-based method invalidates cached images after a specific amount of time since the previous download. This has been set to 4 weeks, initially. Once a podcast cover has been stale for 4 weeks, it is re-downloaded. To get updates earlier, a hash-based mechanism will be implemented. This will hash the image's URI and detect when the URI changes. Usually, the Podcast cover is renamed so this will detect it. The time-based mechanism is a fallback to guarantee updates even if the image updated but retains the same name.

One column is added to the shows table for each caching method. The timestamp image_cached and the hash image_uri_hash.

Testing has yet to be implemented for this functionality.

Fixes #144 (closed)