Plugin Directory: Improve Readme Validator styling

[SergeyBiryukov]

• The validator uses the standard WP notice for the success message, but a regular old-style list for error messages. They should be updated to notices as well.
• Two headings should be combined into one.
• The layout should be visually consistent with ​https://wordpress.org/plugins/developers/ (centered column).

[tellyworth]

First attempt at fixing the markup and css issues

[tellyworth]

attachment:readme-validator-markup-2780.diff​ covers the main issues raised in the ticket.

• Uses better markup and correct styling for errors and notices
• Uses h3 for the smaller headings
• Removed the main dupe heading

Thanks @dufresnesteven for the help!

[dufresnesteven]

Here's a snap based on @tellyworth's diff

[dufresnesteven]

@SergeyBiryukov Anything missing in the diff that needs to be addressed?

[dufresnesteven]

In 9919:

Plugins: Update styling for use in readme validator.

Fixes: #2780

[dufresnesteven]

In 9920:

Plugins: Update readme validator to use updates error notices.

Props tellyworth, sergeybiryukov
Fixes: #2780