* Use new API to fetch videos from channels
This mirrors the process used by subscriptions.gir.st. The old API is
tried first, and if it fails then the new one is used.
* Use the new API whenever getting videos from a channel
I created the get_channel_videos_response function because now instead
of just getting a single url, there are extra steps involved in getting
the API response for channel videos, and these steps don't need to be
repeated throughout the code.
The only remaining exception is the bypass_captcha function, which still
only makes a request to the old API. I don't know whether this code
needs to be updated to use the new API for captcha bypassing to work
correctly.
* Correctly determine video length with new api
* Remove unnecessary line
4a6e920d0e