MDL-75872 mod_lesson: allow unlimited attempts default value.

Related change in 252e85be added a similar value per activity, we
should also allow it to be chosen as a default for the site.

mod/lesson/override_form.php

@@ -223,7 +223,7 @@ class lesson_override_form extends moodleform {
         $mform->setDefault('review', $this->lesson->review);
 
         // Number of attempts.
-        $numbers = array();
+        $numbers = ['0' => get_string('unlimited')];
         for ($i = 10; $i > 0; $i--) {
             $numbers[$i] = $i;
         }

mod/lesson/settings.php

@@ -125,7 +125,7 @@ if ($ADMIN->fulltree) {
         get_string('displayreview', 'lesson'), get_string('displayreview_help', 'lesson'),
         array('value' => 0, 'adv' => false), $yesno));
 
-    $attempts = array();
+    $attempts = ['0' => get_string('unlimited')];
     for ($i = 10; $i > 0; $i--) {
         $attempts[$i] = $i;
     }