Pass correct model through as parent model in tab submodules

In order for submodules to have access to the top-level properties such as `page-type` they need to have the correct parent model passed to the submodules.

The `parent` model should always be the top level model.
Render info section in ppt on a per-tab basis

Info is tab-level rather than tab-set-level. To avoid duplication (by which I also mean "masses of config editing"), copy the info property down from the tab-set to the children.
Add some more tests for tab module

By "some more" I mean "some"
Fix heading selectors for tab submodules

Only want to hide the top-level header, not the other headers inside the more info section in page-per-thing
Merge pull request #585 from alphagov/bugfix/tab-page-per-things

Bugfix/tab page per things