Add in stub method for collapse depth and include in view

As part of a feature to allow users to choose what depth their content should collapse, include a stub method in the presented and include it in the view as a data attribute, that will return an integer describing the depth at which to collapse the headings (either 1 or 2)
Put styling on .js-subsection-title not h2

Since we might be styling h2 or h3 in this way, change the specificity of this to .js-subsection-title.
These styles will not be applied if the js has loaded which is good since we don't want it applied in that event.
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
Merge pull request #102 from alphagov/make-document-collapse-on-correct-thing

Make javascript handle collapsing h3s or h2s