gui: Add id attribute to login button, allows form filling tools to be used (fixes #9200) (#9201)

Add an id attribute to the submit button shown on the login form. This
allows my password manager's form filling function to interact with the
button after filling in username and password (which already have the id
attribute in place).

gui/default/index.html

@@ -369,7 +369,7 @@
               </p>
             </div>
             <div class="col-md-3 text-right">
-              <button type="submit" class="btn btn-default" ng-disabled="login.inProgress" translate>Log In</button>
+              <button type="submit" id="submit" class="btn btn-default" ng-disabled="login.inProgress" translate>Log In</button>
             </div>
           </div>
         </form>