MDL-72927 core: Make Remember username checkbox look like a checkbox

lib/templates/loginform.mustache

@@ -150,7 +150,7 @@
         </div>
         {{#rememberusername}}
             <div class="login-form-rememberusername form-group">
-                <div class="custom-control custom-radio">
+                <div class="custom-control custom-checkbox">
                     <input type="checkbox" name="rememberusername" id="rememberusername" class="custom-control-input" value="1"
                         {{#username}}checked{{/username}} />
                     <label class="custom-control-label" for="rememberusername">{{#str}} rememberusername, admin {{/str}}</label>