Implement infinite scrolling and by date navigation for coursebook

See merge request !355

& do the TODO.
Also leave a pointer how to improve it (even) more.

It does not have to be the currentDate. This holds only for the first day.

Updating the hash fragment causes a small delay while scrolling.
=> Update only after user finished scrolling.
Uses requestIdleCallback
which is not ready for prime time in Safari. But should work elsewhere.

by introducing rootMargin.

app-bar & crud-iterator header where not included in top element
calculation.
165 px is again a magic number & should be replaced by direct
references.
But it is still not clear how to get the app-bar height.

Each day is now a component with a focus method.
The magic number: scrollMarginTop: '145px
Is the height of both headers & should be accessible in
script/template; but isn't.

The state is now in the handler

This version does not rely on internal state anymore.

The first working endless scroll.
But update of visible is broken now.

Leukeju's initial nav bar sketch is superseded.

Seems way more reliable than keeping scroll position (as was done
previously; inspired by the vue static site generator vuepress)

-> no more docsByDay state in component

fetchMore operates on the raw query result
query update fn is then called on the combined result
this result is then propagated through the page

Was loading while scrolling before!