Carry over audio-only mode in playlist links

[krystof1119]

Currently, when a video is viewed in audio-only mode (/watch?listen=1...), the 'related video' links shown on the video include listen=true in the URL parameters, but if the video was opened as part of the playlist, the links to other videos in that playlist don't include it. This PR changes it such that the links have '&listen=1' appended to them when the user is watching a video in audio-only mode.

I made the same change for mixes, and from a cursory look, it seems to me as though it's working (but I personally don't use mixes, so I can't say for sure).

[krystof1119]

Sorry, misclicked and rerequested the review, didn't mean to do that.

[krystof1119]

Sorry for the late response, time got away from me. I've incorporated the suggestion and rebased the patch onto the current version of the code.

[syeopite]

Thank you for your work! Apologies for how long it has taken for this to be merged 😅