Invoice Meta Box text and visual change

[Kenshino]

See ​https://cloudup.com/c2J7xkYnqjx

The 'Edit Existing Invoices' is a misnomer since you can't actually edit submitted invoices. It'd be more like 'See Existing Invoices', or just 'Existing Invoices'

I don't think we need a 'Add New Invoice' title to a button that says 'Add New Invoice'

Visual hierarchy as well - the 2 'section titles' should not be bigger than the meta box title.

Happy to patch if you guys agree

[coreymckrill]

In 5535:

WordCamp Sponsors: Improvements to Invoice metabox

• De-emphasizes the "existing invoices" heading and removes the word "edit" since they can't always be edited.
• Removes the redundant "add invoice" heading.

Note that I didn't remove the link to the existing invoice in the list,
like the original patch had, since it can still be useful to view the
invoice details, even if they can't be edited.

Props SergeyBiryukov
Fixes #1831