Make the calendar filter use up less space on the main calendar page

Currently the filter introduced in MOCCACAL-16 can take up quite a bit of space if there are many calendars (some folks seem to have easily 20+ calendars).

It would be nice to make the filter use up less space so they do not have to scroll when looking at the calendar.

Ideas:

1. instead of the list of checkboxes use a "page selector" like for the pinned blog posts.  (As most calendars might be simply named "Calendar", an the navigation path provides the context, this might not always be optimal - but then the drop down to select a calendar when creating an event has the same issue.)
2. Borrow, or just use the "show/hide" macro to hide the filter normally, and only show it when calendars got selected for filtering.

Also it might be possible to jump down to the actual calendar view if the form is submitted (by appending #calendar0 to the form action) - I think there should be no other calendars on the main calendar page.