Add option for data clouds (like Nextcloud)

Current behavior
Currently it is not possible to access a data cloud like Nexcloud via OpenSlides easily. Indeed, it is possible to add links onto the meeting starting page or in some other sub-pages, but this is not easily accessible. It should have an easier way to reference a cloud which is used by the users of a meeting.

Wanted behavior
A good first option for integrating a cloud would be to make it available as a link with an individual icon in the meeting navigation bar. Clicking on the link would then open the corresponding cloud in a new tab, which would have the advantage that the link would be visible at all times.

What is needed for this?
A new option should be integrated in the meeting settings > general, which enables a) a freely selectable title, b) the setting of a link and c) the selection of an icon. Once you have entered and saved everything, the title with icon and link will be displayed as the last item in the navigation menu.

The icon base can be icons from the material design.

Start by tracing the meeting settings > general flow and the meeting navigation menu. Identify how settings are saved, how navigation items are rendered, and how Material Design icons are selected. Done means users can save a title, link, and icon, see it as the last navigation item, and open the cloud in a new tab.