Add JS to choose which heading level to collapse on

We're allowing publishers to choose which heading level to make collapsible, so this commit updates the javascript to:

1. Read the heading depth from a data-attribute
2. Calculate the selector for that level (i.e. convert 1 to 'h2', 2 to 'h3' n to 'hn+1') Defaults to 'h2' if there is no depth value provided
3. Calculate the selector for 'this heading and any heading levels bigger' to use when working out where the end of a collapsible section is
4. Update markupSections() to use new selectors
Make stub method return 1

1 equates to "collapse on h2" which is the current behaviour in the published manuals.
Add a test for calculateSuperiorsSelector

Not the most ingenious test, but hey ho. I don't think there is a much more flexible way to test this (rather small) function