Scope scroll-depth to only `#maincontent`

Instead of using the entire page scroll depth, do some fancy math to determine scroll depth of `#maincontent`. This avoids potential data distrust caused when users don't scroll the entire footer, for example.
Remove baseline tracking, it distorts the averages.
Remove unnecessary function.
Merge pull request #438 from smeranda/scroll-tweaks

Setup scroll-depth tracking to only focus on `#maincontent`