Focus error summary on window load

[NickColley]

This adds functionality to focus the error summary component when the page loads.

This helps users of assistive technologies recognise and get out of situations when they've encountered an error in their submission.

This re-implements functionality in GOV.UK Elements, but avoids re-implementing the logic to set event.preventDefault() on links clicked.

It's not clear why this was done, my assumption is that it's done to avoid cluttering the URL with anchor hashes but I'm not including this as we're not sure.

Testing

Browser testing

Testing with browsers (✅)

This table is based on https://www.gov.uk/service-manual/technology/designing-for-different-browsers-and-devices#browsers-to-test-in

Operating system	Browser	Result
Windows	Internet Explorer 8-10	✅
	Internet Explorer 11	✅
	Edge (latest versions)	✅
	Google Chrome (latest versions)	✅
	Mozilla Firefox (latest versions)	✅
macOS	Safari 9+	✅
	Google Chrome (latest versions )	✅
	Mozilla Firefox (latest versions)	✅
iOS	Safari 9.3+	✅
	Google Chrome (latest versions	✅
Android	Mozilla Firefox (latest versions)	✅
	Google Chrome (latest versions)	✅
Windows Phone	Internet Explorer (latest versions)	✅

Accessibility

Testing with colours overriden (N/A)

This is based on https://accessibility.blog.gov.uk/2017/03/27/how-users-change-colours-on-websites/

N/A

Testing with assistive technologies (✅)

This table is based on https://www.gov.uk/service-manual/technology/testing-with-assistive-technologies

Software	Version	Browser	Result
JAWS	15 or later	Internet Explorer 11	✅ Reads out alert heading
ZoomText	10.11 or later	Internet Explorer 11	N/A
Dragon NaturallySpeaking	11 or later	Internet Explorer 11	N/A
NVDA	2016 or later	Mozilla Firefox (latest versions)	✅ Reads out entire alert, and announces the component as a landmark
VoiceOver Mac OSX	N/A	Safari 10 (or higher) on Mac OS X 10.10	✅"You are currently on an alert" Same behaviour as GOV.UK Elements
VoiceOver iOS	N/A	iOS 10.3 (or higher)	✅

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https://trello.com/c/UzvIbXmM/775-2-automatically-focus-the-error-summary

[alex-ju]

💯 Look good!

We should settle on a testing template and standardize it for contribution. This PR message seems like a good starting point.