Wp-calypso: AT: update eligibility screen design

[davewhitley]

As discussed in #11394, we need to update the eligibility error page design.

Current:

After:

https://github.com/Automattic/wp-calypso/issues/11394#issuecomment-280146123

cc @vindl

[davewhitley]

More explanation:

I think this could be done it 2 separate PRs.

1. Update styling and text, no functional changes
2. Update Business gating behavior, explained below

New business gating behavior

Currently if you do not have business or a custom domain, then this shows up as an error:

Unfortunately this is problematic because the user could buy a domain first, without knowing that you get one free with business. Also, the red errors make this _positive_ thing into a scary thing.

We should treat the business error and domain error separately. In this case, the user has no business plan and no domain. Notice that the domain error is not shown.

If they get business without setting their free domain, we should show them this once they try AT again. This banner is shown if they have not user their domain credit:

Theme uploading does something similar, but the errors are still listed even though the business banner is shown:

[folletto]

An implementation note: the text in the Banner:

Please update to install this plugin

Might be necessary to be either an external parameter or changed to be generic as we use it in both Plugins and Themes. :)

[davewhitley]

Recently learned that if a user buys a domain, then upgrades to business, they get a discount on the business plan. So, they wouldn't waste money if they bought the domain first, then the plan.

Might still be worth implementing the flow in the mockups though.

[Copons]

Closed after #11575 merge.