[Do not merge] Update govuk_template to 0.20.0

[fofr]

• The tweak reported here: Changes to template header search will need minor change to static #848, no longer appears necessary: Changes to template header search will need minor change to static #848 (comment).
• Chrome doesn't handle the ink style links nicely for y characters: [Discuss] Add text-decoration-skip: ink to all links on GOV.UK govuk_template#281 (comment) (but letters like p and g are more readable)

https://github.com/alphagov/govuk_template/blob/master/CHANGELOG.md

0.20.0

• Fix EJS template (PR #270)
• Add theme-color support to make the page surround in Chrome’s tab view on Android match the black GOV.UK header (PR #278)
• Add text-decoration-skip: ink to all links on GOV.UK (PR #281)
• Improve contrast of links when focused (PR #272)
• Make header text colour black when focused (PR #274)

0.19.2

• Increase skiplink colour contrast (PR #263)
• Fix Scala compile issues for Play template (PR #261)

0.19.1

• Have focus outline appear outside of element rather than covering it in Safari and Chrome (PR #259)

[fofr]

Dupe.