Better handling Show Update Status "Ended"

Since the "Ended" Status is set on the tvdb many times as soon as the TV Shows is canceled, there are still Episodes left to air. So stopping to do Updates for "Ended" Shows is not a good idea. I changed that.

Now the last known Airdate and the the Airdate of the last Episode is read from the db. If a Show is marked "Ended" the Show is still updated if:

1. The last Episodes Airdate is known = up to 90 days after the last known Airdate a normal Update is done.
2. The last Episodes Airdate is unknown = up to 365 days after the last known Airdate a normal Update is done.

Both is necessary to avoid the need of a manual Forced Update for Shows that are marked "Ended", but not all Episodes have been aired yet.