Add a tool to lint Stagecraft JSON
Run linter on Stagecraft JSON

This has highlighted a couple of embarrassing inaccuracies in our stub
data, including one page where we are showing incorrect data.

Where I have made changes, I have used the dashboard as the true source of
data, as I assume it has had more eyes on it.

Visible changes:

- Fix data swap on pay-register-{birth,death}-abroad service-availability pages (!)
- Fix description on GOV.UK dashboard
- Fix period and title on housing/average-first-mortgage page-per-thing
- Fix description on 1 housing page-per-thing

No-op, stub improvements:

- Add several blank descriptions where they don't exist
- Change 1 absolute link to relative (almost a no-op)
- Fix 3 dashboard-straplines
- Add matching-attribute to main FCO dashboards (being explicit about default)
- Remove _start_at from g-cloud dashboard (no-op as far as I can tell)
- Remove 7 slugs from page-per-thing pages
- Add cumulative false to 1 housing page-per-thing (no-op?)
- Improve no-realistic-dashboard