Let's Read v2.89

Fixes to Let's Measure that make it compliant with Drupal 9 content.

In case a teacher object loses references to classes, an automatic process recovers any references tied to existing class directories.

In case a school loses references to teachers, a process on teacher select recovers any references tied to existing teacher directories.s

When going from a slide back to lesson select, the screen slides to the right rather than to the left.