Rescrape season art when new season are added

This adds a GetArt() function to the scraper, allowing to (re)fetch the art URLs for a particular item.  Currently supports tvshows, though it should work for movies well enough.  It's currently not ideal for shows, as it requires an additional XML fetch atm (banners.xml) as we don't have the language used for initial scrape when we re-fetch art.

Requires a unique ID to be set in the appropriate field in the video database.  Ideally anything else required to get a direct hit (e.g. language) would be added by scrapers initially.

This fixes a bug (season art not available for new seasons) as well as assisting to support in the future refetching of art URLs whenever the user goes to "Get Art" in the UI.  i.e. no need to store URLs that will almost certainly expire at some point: Instead we have an id that can be used to (re)generate new art.  Also means art doesn't become stale (scan when movie first comes out -> limited art, scan a year later -> more art).

ATM hooked up only for regenerating season art when we hit a season not previously discovered.  Will look at extending to make sure we have the framework in place at the scraper level to do what we want to in future.

Scraper devs, please comment on what else you might need to be able to pull this off properly, particularly for scraper sites outside the big two (tvdb, themoviedb).