fix: youtube: Polymer UI and JSON endpoints for playlists

fix: youtube: Polymer UI and JSON endpoints for playlists

We already had a few copies of Polymer-style pagination handling logic
for certain circumstances, but now we're forced into using it for all
playlists since we can no longer disable Polymer. Refactor the logic to
move it to the parent class for all entry lists (including e.g. search
results, feeds, and list of playlists), and generify a bit to cover the
child classes' use cases.