AP-705 (revisited) Attempt to better customize webhint

[MalcolmVonMoJ]

What

Link to story

Customized the webhint .hintrc file to allow for specific bypassing of hints that the GDS desing system will fail.
Confirmed what messages are okay with Oliver Byford at GDS and from the GDS list of known validation errors

Checklist

Before you ask people to review this PR:

• Tests and rubocop should be passing: bundle exec rake
• Github should not be reporting conflicts; you should have recently run git rebase master.
• There should be no unnecessary whitespace changes. These make diffs harder to read and conflicts more likely.
• The PR description should say what you changed and why, with a link to the JIRA story.
• You should have looked at the diff against master and ensured that nothing unexpected is included in your changes.
• You should have checked that the commit messages say why the change was made.