[ticket/13291] Notification drop down auto close

Notification drop down should auto close once a user clicks on "mark all
read"

PHPBB3-13291
This should use `phpbb.toggleDropdown()` instead, otherwise the dropdown is not properly closed. Some of the classes that are toggled when the dropdown is toggled will not be updated. This causes some strange behaviour. For example, click "Mark all read," let the alert close by itself, then click anywhere on the page. The dropdown will open.

The `phpbb.toggleDropdown()` method needs an optional parameter to be added that will be used in place of `this` so that you can use it in this instance. Then you can pass the DOM object for `#notification_list_button` to it.