Remove the word 'key' from metrics

It's on the banned words list.
Remove colour from need roles, goals and benefits

The feedback from content designers (who are the target audience)
was that the colours made the need definition harder to read.
Improve need justification microcopy.
Improve need label microcopy.
Break up role/goal/benefit over 3 lines

Feedback from the users was that it's harder to read when the three
parts are smushed together, especially when one of them is very long.
Readability tweaks
Add link to the GOV.UK proposition
Merge pull request #12 from alphagov/fixes-after-content-design-review

Fixes after content design review