fix: prefer HTML5 video when youtube support is deprecated.

In the case where a server/course has both:

* YouTube deprecated (waffle flag: videos.deprecate_youtube = True)
* HLS enabled (ConfigModel: HLSPlaybackEnabledFlag, in video_config)

Prior to this commit, we would prefer the HLS source if one of the
HTML5 video sources specified a .m3u8 file. However it looks like this
file isn't always guaranteed to be produced.

This fix always prefers HTML5 video sources to YouTube if the server or
course is configured to deprecate YouTube, as long as at least one
non-YouTube source is found. TNL-8594.

Note that this may confuse users because we're essentially bypassing
their Studio-specified preferred primary source with a server-wide
setting.