MDL-75083 login: prevent double submission

Apply core_form/submit JavaScript code to prevent double submission of login form and guest login button.
Add ID to guest login button to make it addressable.

lib/templates/loginform.mustache

@@ -180,7 +180,7 @@
             <input type="hidden" name="logintoken" value="{{logintoken}}">
             <input type="hidden" name="username" value="guest" />
             <input type="hidden" name="password" value="guest" />
-            <button class="btn btn-secondary" type="submit">{{#str}}loginguest{{/str}}</button>
+            <button class="btn btn-secondary" type="submit" id="loginguestbtn">{{#str}}loginguest{{/str}}</button>
         </form>
     {{/canloginasguest}}
     <div class="login-divider"></div>
@@ -213,4 +213,8 @@
     {{#error}}
         document.getElementById('loginerrormessage').focus();
     {{/error}}
+    require(['core_form/submit'], function(Submit) {
+        Submit.init("loginbtn");
+        Submit.init("loginguestbtn");
+    });
 {{/js}}