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.

_includes/episode_title.html

@@ -2,7 +2,7 @@
     Find previous and next episodes (if any).
 {% endcomment %}
 {% for episode in site.episodes  %}
-  {% if episode.title == page.title %}
+  {% if episode.url == page.url %}
     {% unless forloop.first %}
       {% assign prev_episode = prev %}
     {% endunless %}