Fix existing AMP validation errors

[radiocontrolled]

Describe the bug
There are existing AMP validation errors that need to be fixed before #442 can be completed.

To Reproduce
Steps to reproduce the behavior:

1. npm run build
2. npm run start
3. in a separate terminal window, do npx amphtml-validator http://localhost:7080/news/articles/c9rpqy7pmypo.amp || true
4. See error

Errors

http://localhost:7080/news/articles/c9rpqy7pmypo.amp:1:3610 The mandatory attribute 'amp-custom' is missing in tag 'style amp-custom'. (see https://www.ampproject.org/docs/guides/author-develop/responsive/style_pages)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:53:2530 The tag 'head > style[amp-boilerplate] - old variant' appears more than once in the document. (see https://github.com/ampproject/amphtml/blob/master/spec/amp-boilerplate.md)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:290 The attribute 'focusable' may not appear in tag 'svg'. (see https://www.ampproject.org/docs/reference/spec#svg)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:3048 The tag 'img' may only appear as a descendant of tag 'noscript'. Did you mean 'amp-img'? (see https://www.ampproject.org/docs/reference/components/amp-img)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:4477 The tag 'img' may only appear as a descendant of tag 'noscript'. Did you mean 'amp-img'? (see https://www.ampproject.org/docs/reference/components/amp-img)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:5993 The tag 'img' may only appear as a descendant of tag 'noscript'. Did you mean 'amp-img'? (see https://www.ampproject.org/docs/reference/components/amp-img)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:7584 The tag 'img' may only appear as a descendant of tag 'noscript'. Did you mean 'amp-img'? (see https://www.ampproject.org/docs/reference/components/amp-img)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:8736 The tag 'img' may only appear as a descendant of tag 'noscript'. Did you mean 'amp-img'? (see https://www.ampproject.org/docs/reference/components/amp-img)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:10363 The attribute 'focusable' may not appear in tag 'svg'. (see https://www.ampproject.org/docs/reference/spec#svg)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:14676 Custom JavaScript is not allowed. (see https://www.ampproject.org/docs/reference/spec#html-tags)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:30418 Custom JavaScript is not allowed. (see https://www.ampproject.org/docs/reference/spec#html-tags)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:30542 Custom JavaScript is not allowed. (see https://www.ampproject.org/docs/reference/spec#html-tags)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:30666 Custom JavaScript is not allowed. (see https://www.ampproject.org/docs/reference/spec#html-tags)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:30790 Custom JavaScript is not allowed. (see https://www.ampproject.org/docs/reference/spec#html-tags)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:30921 The mandatory tag 'amphtml engine v0.js script' is missing or incorrect. (see https://www.ampproject.org/docs/reference/spec#required-markup)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:30921 The mandatory tag 'noscript enclosure for boilerplate' is missing or incorrect. (see https://github.com/ampproject/amphtml/blob/master/spec/amp-boilerplate.md)
http://localhost:7080/news/articles/c9rpqy7pmypo.amp:55:30921 The mandatory tag 'noscript > style[amp-boilerplate]' is missing or incorrect. (see https://github.com/ampproject/amphtml/blob/master/spec/amp-boilerplate.md)

Expected behavior
No errors

[ChrisBAshton]

The attribute 'focusable' may not appear in tag 'svg'. (see https://www.ampproject.org/docs/reference/spec#svg) is logged in issue #679.

[ChrisBAshton]

Have worked on a proof of concept fix here: #878

Raised each individual validation error in #883, #886, and #885.

A wider investigative piece of work will need to be done to handle component-level AMP requirements - captured here: #884.