MDL-85902 course: Better labeling of activity navigation components

* The arrow icons produced by the `$OUTPUT->larrow()` and
`$OUTPUT->rarrow()` functions are announced by screen readers, and they
do not make sense when heard by screen reader users. To fix this,
we need to override the accessible names of the previous and next
activity links using the `aria-label` attribute and specify whether the
activity links point to the previous or next activities.
* The label for the jump menu "Jump to" is not informative enough to
convey the element's purpose. Let's improve it's accessible label to
"Jump to activity".

Author: junpataleta

course/classes/output/activity_navigation.php

@@ -74,6 +74,7 @@ public function __construct($prevmod, $nextmod, $activitylist = array()) {
             $attributes = [
                 'class' => 'btn btn-link',
                 'id' => 'prev-activity-link',
+                'aria-label' => get_string('activitynavigation:preva', 'course', $linkname),
             ];
             $this->prevlink = new \action_link($linkurl, $OUTPUT->larrow() . ' ' . $linkname, null, $attributes);
         }
@@ -89,14 +90,15 @@ public function __construct($prevmod, $nextmod, $activitylist = array()) {
             $attributes = [
                 'class' => 'btn btn-link',
                 'id' => 'next-activity-link',
+                'aria-label' => get_string('activitynavigation:nexta', 'course', $linkname),
             ];
             $this->nextlink = new \action_link($linkurl, $linkname . ' ' . $OUTPUT->rarrow(), null, $attributes);
         }
 
         // Render the activity list dropdown menu if available.
         if (!empty($activitylist)) {
-            $select = new url_select($activitylist, '', array('' => get_string('jumpto')));
-            $select->set_label(get_string('jumpto'), array('class' => 'sr-only'));
+            $select = new url_select($activitylist, '', array('' => get_string('activitynavigation:jumptoactivity', 'course')));
+            $select->set_label(get_string('activitynavigation:jumptoactivity', 'course'), ['class' => 'sr-only']);
             $select->attributes = array('id' => 'jump-to-activity');
             $this->activitylist = $select;
         }

lang/en/course.php

@@ -41,6 +41,9 @@
 $string['activitydate:opened'] = 'Opened:';
 $string['activitydate:opens'] = 'Opens:';
 $string['activitynavigation'] = 'Activity navigation';
+$string['activitynavigation:nexta'] = 'Next activity: {$a}';
+$string['activitynavigation:preva'] = 'Previous activity: {$a}';
+$string['activitynavigation:jumptoactivity'] = 'Jump to activity';
 $string['aria:coursecategory'] = 'Course category';
 $string['aria:courseshortname'] = 'Course short name';
 $string['aria:coursename'] = 'Course name';