Bug: Reverting playlists breaks YouTube mixes

[Rainyan]

Context

Mixes are playlists auto-generated by YouTube for the user.

Description

If you use the Iridium "Reverse playlist" button to revert a playlist, and then navigate to a YouTube Mix playlist, the mix playlist will be reverted, but start playing from the last entry. This means that the Mix will stop playing after the first (last) entry and autoplay will not occur. This effectively breaks YouTube Mixes for things like background music playlists until the user goes to a normal playlist and undoes the Iridium "Reverse playlist" selection, and then re-loads the Mix.

Steps to reproduce

• Navigate to a normal YouTube playlist page
• Use the Iridium "Reverse playlist" button to revert it
  • If the button is missing, see Reverse Playlist Button Missing. #1017 (comment) for a workaround
• Navigate to a YouTube mix playlist page
  • You can typically find these mixes recommended either in your main YT page, or the sidebar of a video page
• Fast-forward to the end of the first video in the Mix playlist

What should happen

• The YouTube mix begun playback at the first video of the Mix playlist
• When the first video ends, the next video in the mix will start (if autoplay enabled)

What actually happens

• The YouTube mix is incorrectly reverted, and the first video has become the last
• When the first video ends, autoplay will not kick in, because we're already at the end of the list

Alternatives considered

Alternative 1

Make playlist reversion a per-playlist action that doesn't affect any other playlists.

Alternative 2

The "Reverse playlist" feature could also apply to Mixes, such that they also reversed their order correctly. But maybe this should be its own feature, as dynamically generated mixes and hardcoded playlists are somewhat different at least conceptually.