Add and improve print stylesheets for manuals on gov.uk

• create print scss file to compile print styles
• add print styles for manuals -
  • remove unecessary page elements for print
  • reduce and rationalise font sizes and weights
  • remove underlines from link as they are unhelpful when printing
  • shorten line lengths to 75% page width for better reading
  • improve general spacing for clarity and ease of reading
Write specs for existing behaviour
Attempt to serve employment-income-manual

If it exists in content-store, then show it as normal. If it doesn't, show the
holding page as before.

This is useful because the employment-income-manual is the one that the HMRC
Manuals API team use, and also that HMRC/their developer iO1 use too. This
should also remove a small burden when it comes time for this manual to be
published for real.
Merge pull request #83 from alphagov/manuals-print-stylesheets

Add and improve print stylesheets for manuals on gov.uk
Display manual body if its available
Merge pull request #87 from alphagov/make_employment_income_manual_try_content_store_first

Make employment income manual try content store first
Merge pull request #86 from alphagov/feature/manuals-related-links

Display manual body if its available
Add some styling around newly included body

Mostly just add some gutters around new body copy so that the summary and the body don't run in to one another
Merge pull request #88 from alphagov/related-links-for-manuals

Add some styling around newly included body