reduce whitespace around H5s and H6s in HTML pubs

- keep top-margin above H5 & H6s to 30px in wider viewports
- strip whitespace between H5 & H6s and P, UL, OL elements
Reduce whitespace above H4s

- remove the 80px margin-top from H4s in wider viewports
override default i/em/b/strong styles
Merge pull request #647 from alphagov/h5-h6-spacing

H5 h6 spacing