Create new-style start page for check-vehicle-tax

Use new style start page and link to the beta vehicle enquiry service
Add the usage requirements to the original service
Swap requirements and instructions
Use style guide for requirements heading
Widen the "you'll need" column
add mobile margin
Change related links title to match browse page
Change service URL for check vehicle tax
mobile spacing, you'll need
Merge pull request #578 from alphagov/view-driving

mobile spacing view driving licence
Remove exception notification
Merge pull request #577 from alphagov/check-vehicle-tax-start-page

Check if a vehicle is taxed start page
Remove focus on <main> element

Having a negative tab index allows the user to focus on the main element,
which adds an outline from a user agent stylesheet. This was added in
https://github.com/alphagov/frontend/commit/cd20d6ec2dab5fff2282a60cf1152173030bd5dd
but the 'Skip to content' links behaviour is unchanged in this commit.
Only import toolkit buttons mixin once
Merge pull request #581 from alphagov/refactor-buttons-mixins

Only import toolkit buttons mixin once
Remove mustache dependancy

It was needed when there was ajax for the frontend but as that is no
longer used it can be removed.
Use title/URL from publisher for new start pages

We're currently hard-coding these in the templates, but there are some
scenarios where it might be useful to have control of them via publisher
Merge pull request #583 from alphagov/link-new-startpages-to-publisher

Use title/intro/URL from publisher for new start pages
Merge pull request #582 from alphagov/remove-mustache

Remove mustache dependancy
remove obsolete email-content testing for exceptions