Publicize: add sharing preview modal

[jhnstn]

This adds a preview modal to publicize:

breakpoint > 660px

breakpoint < 660px

Note
At the moment we only have previews for Facebook and Twitter. #12644 adds the remaining available services.

Testing

• Open sharing option on a post from http://calypso.localhost:3000/posts/my/:site
• Edit customized message
• Click "Preview"
• The preview modal should open
• The preview should show the customized message
• Click the close modal button ( top left X )
• The preview modal should close

[matticbot]

Test live: https://calypso.live/?branch=add/publicize-preview-modal

[gwwar]

Thanks @jhnstn changes here look good and test well

[gwwar]

This is optional, but if we're concerned with pass through props, perhaps:

{ children }

So in the caller this looks like:

<SharingPreviewModal isVisible={ isVisible } onClose={ onClose }>
    <SharingPreviewPane siteId={ siteId } postId={ postId } message={ message } />
</SharingPreviewModal>

[gwwar]

If we pull out the sharing specific props, this component becomes reusable if we need a dialog that looks like a web preview. (We'd need to change class names though and move this into the component folder.)

[matticbot]

@jhnstn This PR needs a rebase

[jhnstn]

closing in favor of #12753