Add/republicize preview

[jhnstn]

This adds a preview modal to publicize and updates share action button styles.

Action buttons

breakpoint > 480px

breakpoint < 480px

Modal
breakpoint > 660px

breakpoint < 660px

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

Testing

• build with ENABLE_FEATURES=publicize/preview make run
• 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/republicize-preview

[gwwar]

Copying over notes from the other PR:

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.)

[retrofox]

yup, it's a good idea. I think we should move this component into its folder straight in this PR.

[gwwar]

I don't mind waiting for a follow up PR to do this change though

[jhnstn]

I'll land this now and do the follow up extraction in another PR.

[gwwar]

Tested this enabled/disabled 👍 Let's 🚢 this for now. Do you mind following up on making the modal a generic component in another PR after this lands?

[retrofox]

Looks pretty good Jason.

I wonder if we should consider the case when the user has one more account of the same media.
In the screenshot above I have two twitter accounts. I think it could/should be defined after this implementation.

[gwwar]

I wonder if we should consider the case when the user has one more account of the same media.

For a first pass I think taking the first enabled account works.