Prevent showing videos that aren't in the app config

[dbudzins]

Currently you can add any media ID to the URL after the /m on the media page and it will show that video on any site even if it's not from the given property.
https://app-preview.jwplayer.com/m/awWEFyPu/big-buck-bunny?r=dGSUzs9o&app-config=a2kbjdv0

Since we have the app config, it should be easy enough to verify that the video is defined in one of the playlists and if it's not, show an error screen instead of the video.

It would be nice if this was an optional param. It could even be a custom setting on the app config itself. Still need to figure out how to handle search and recommendations playlists and series.