ENTST-139 Add Enterprise Sharing to FTR Articles

[willhoward]

This PR tweaks the behaviour of x-gift-article to make a few adjustments concerning Enterprise Sharing. The main difference is that when sharing a free to read article with Enterprise Sharing enabled, users will now see the option to share with Enterprise Sharing alongside the default share option.

This is necessary because articles commonly remain free to read for a short period of time before going behind the paywall and we've had feedback from several Enterprise Sharing clients expressing frustration that the standard links to the article they've shared stop working when this happens. Enterprise Sharing clients can also track how many times their links have been opened and therefore some would prefer to share free to read articles via Enterprise Sharing to have visibility over this.

This behaviour is covered by ticket ENTST-139.

I've added a 'Free with enterprise sharing' option to the Storybook to make this behaviour available to test.

Styling Tweaks

This PR also makes a few minor styling changes to the x-gift-article:

• The 'Enterprise' label has been removed from next to the 'Enterprise Sharing' radio button. Ticket ENTST-152.
• The sharing link url field label has been corrected to read 'Enterprise Sharing link' instead of 'Gift article shareable link' when the link has been generated with Enterprise Sharing. Ticket ENTST-154.
• A 'View all Enterprise Links' link which takes users to the Enterprise Sharing Dashboard has been added to the messaging when the Enterprise Sharing option is selected. Ticket ENTST-135.

[willhoward]

I think there are some undesired side-effects in these changes:

1. Button text has now the same colour as background
2. I'd also suggest add a spacing between the label and the warning icon

Thanks, these are now fixed