Match pages by URL rather than title when constructing next/prev links.

Earlier version was matching episode.title with page.title to find the episode object for a page when generating next/prev links.
This fails when several episodes have the same title (e.g., "Morning Coffee" for a two-day workshop schedule).
This fix matches episode.url with page.url instead.

This fixes https://github.com/swcarpentry/instructor-training/issues/160.