Do not display calendar events in the page navigation tree

As mentioned in MOCCACAL-112 all events of a calendar are shown in the navigation tree, which might make the navigation branch quite large. Currently this is mitigated as the navigation tree supports some kind of paging, but it might be preferable not to show them at all.

(Keeping this as a reminder to review the situation in case a good solution pops up. As calendar events will the no-terminal pages from 2.6 on, it might actually make sense to show them in the navigation tree as long as they have sub pages.)