1.  Put prev/next finding code into separate include.
2.  Modify episode_title to use that.
3.  Include it in the footer and add prev/next arrows if there's a match.

Note: the prev/next finding code must be added to *both* episode_title and footer because variables set in includes aren't exported to other includes.

This closes #1.

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.