This fixes the crash on login when you first visit the timetable screen.

This was added in
3d7cce6#diff-92485d835e426b7a4ef0a643b8c946ff79948ad6388ef0089a9ba6907ad6562dR62
but I can't get the login to work without removing it, and the docs seem
to say that isn't required. It's working on Ethan and my device with
this change now.

This was happening when initialScrollIndex > 0 and the cell hadn't
rendered yet.

We'll now try to re-scroll after a slight delay to allow the cell to
render.

This also changes the behaviour of auto-scrolling so that only if you
scroll into the current week will the page auto-scroll to the current
day. Previously, all weeks started at the current day (e.g., today is
Tuesday, scroll to next week, and the first day is still Tuesday).