Update Image to include AmpImg & add srcset prop

[sareh]

Part of bbc/simorgh#929

• Add AmpImg, update Image to include srcset prop

Testing notes
When you visit storybook, you should see two sets of stories in the menu. Image - AmpImg and Image - Img.
The AmpImg stories should load correctly, with all associated Amp HTML.

http://localhost:8080/?selectedKind=Image%20-%20AmpImg

• I have assigned myself to this PR and the corresponding issues
• Tests added for new features
• Test engineer approval

[sareh]

Bug found: When running storybook npm install && npm run storybook and visiting an Amp Img story, http://localhost:8080/?selectedKind=Image%20-%20AmpImg&selectedStory=16%3A9%20landscape%20image it loads, but clicking through to a second or a thirds Amp Img story fails to render the <amp-img> image. When the page is refreshed, or if the link is visited directly, e.g. http://localhost:8080/?selectedKind=Image%20-%20AmpImg&selectedStory=image%20with%20srcset, the image does render.

I'd suggest making this a separate issue to this PR.

[ChrisBAshton]

Good start, thanks Sareh

[jamesbrumpton]

LGTM 👍