Use page overflow to make better use of (fast) native scrolling behaviour

Also works better with mobile chrome which hides the navigation bar on scroll
Scroll to top when page changes
Added test for isSaveable, closes #81
More tests for scoresheet
removed inc and dec methods, they were not used (used to be for simple number input control iirc)
tests for score calculation
Made challengeMock a factory
Completed testing of scoresheet (loading and processing)
Add messages to show what is wrong when scores cannot be saved