MDL-67410 Enrolments: Casting the perpage parameter to INT

On the course's Participants page, when manually enrolling users
the AJAX call might get excesively slow, due to the treatment
of the perpage parameter as a string, which gets concatenated with the
number 1 (perpage: parseInt(perpage) + 1) instead of performing a
numerical addition.

If there are for example 1500+ users on the site, the method
core_enrol_get_potential_users will be executed with a value of 15001
for the perpage parameter, rendering painfully slow.

enrol/manual/amd/src/form-potential-user-selector.js

@@ -54,8 +54,8 @@ define(['jquery', 'core/ajax', 'core/templates', 'core/str'], function($, Ajax,
             if (typeof enrolid === "undefined") {
                 enrolid = '';
             }
-            var perpage = $(selector).attr('perpage');
-            if (typeof perpage === "undefined") {
+            var perpage = parseInt($(selector).attr('perpage'));
+            if (isNaN(perpage)) {
                 perpage = 100;
             }