Define custom excerpt length

** Why are these changes being introduced:

* WordPress has a default limit for page excerpts of 55 words, broken on
  a word boundary. This is frequently too short of a snippet to give a
  good indication of page content.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/pw-60

** How does this address that need:

* This defines a custom function to extend the length of the excerpt to
  9,999 words - which is longer than any page on our site should be.
  With this change, the opengraph tag `og:description` in our site
  headers should be a copy of all text in the page body.

** Document any side effects to this change:

* This makes the notion of an "excerpt" kind of meaningless, as it just
  repeats the full body copy.

web/app/themes/mitlib-parent/functions.php

@@ -494,6 +494,17 @@ function customize_body_class( $classes ) {
 }
 add_filter( 'body_class', 'Mitlib\Parent\customize_body_class' );
 
+/**
+ * Defines a new value for the length of the excerpt.
+ *
+ * @param int $length Number of words to include in the excerpt.
+ * @return int The maximum number of words to include from the excerpt.
+ */
+function custom_excerpt_length( $length ) {
+	return 9999;
+}
+add_filter( 'excerpt_length', 'Mitlib\Parent\custom_excerpt_length', 999 );
+
 /**
  * ============================================================================
  * ============================================================================