Anchor scroll offset

Let's keep it simple for now and don't do an integration test, as this would also make the test simpler. We did the same for `$browser`, and so far it turned out to be a good thing.

But yes, we should create a new issue to add e2e tests (reference #9527 in it) for the whole `$anchorScroll` service, where we also test the scrolling itself, and not just the calculation of the scroll position.