perf(raw_update.py)!: Don't fetch entries that are already in the database

Testing has resulted in a speed-up of circa 3400% (updates of 1
subscription (which is already fully stored) are now ca. 1 sec from
previously 34 sec).

BREAKING CHANGE: The extractor hash is now calculated from the `id` and
not the `webpage_url` field requiring a complete re-fetch of all stored
videos.
```bash
$# export your subscriptions:
$ yt subs list --urls > subs.txt
$# remove the old database
$ mv ~/.local/share/yt/videos.sqlite{,.old}
$# reimport the subsciptions
$ yt subs import subs.txt
$# refetch all videos
$ yt upadate
```

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