Check load_more_widget_html for feed paging

I believe the fix for Youtube feeds introduced in f6177462dbba73f80df52a72c666ca769bea173f (closing: #2671) caused an issue with pagination for some(all new-style?) feeds.

Bug found while trying to download my watch later playlist, the playlist has ~150 entries, but only the first page of 100 videos was being downloaded.

The new feeds appear to have the old feed_html split into 2 parts, 'content_html' and 'load_more_widget_html'. 'content_html' was fixed in f6177462dbba73f80df52a72c666ca769bea173f, but the 'load_more_widget_html' is not being used for finding the pagination data.