Add new lastwatchedid column to shows table.
Use last watched episode to determine next episode.

If there is none, choose the oldest unwatched episode.
Modify upgrade code to prepopulate latest watched fields.
Make sure we get mid-season specials on upgrading, too.
Look only at one shows episodes.
FlagTask part one: set last watched if adding watched flag.
FlagTask part two: set last watched if removing watched flag.
Do not prepopulate last episode field after upgrading.

We will show the oldest unwatched episode as next again, this should be
a good hint for users that the algorithm has changed (plus change log).
On updating from trakt set the latest watched id.
Merge branch 'better-next-episode' into beta