Use proportional form width classes

- Match these to grid width styles
Move grid highlight scss file to grid example folder
More form example styles

- Radio buttons
- Checkboxes
- Compound form controls
- Indented content
Updates to form example pages

- Create sample transaction page
- Reduce size of form headings
- Increase spacing between form groups
Merge pull request #11 from alphagov/more-form-styles

More form styles