This simplifies code for callers of `_find_jwplayer_data()` which no longer have
to run `_parse_json()` on the return value.
It also makes sure that `_find_jwplayer_data()` returns either a `dict` or
`None` and nothing else.
discourse.ubuntu.com has gone away, repalce with skiplagged.com.
Be nice to have a non-frontpage URL that might be more stable,
though I don't have one. Maybe this should move to html
in test/test_InfoExtractor.py?