Print buttons use pro print

[mnelson4]

Like #326, but for print buttons. Best done after that.

motivation

• something I can sell to 90% of free users
• add print buttons to other CPTs
• better plugin integration than quick print
• Better UX for visitors (download PDF in one click instead of generating the PDF in their browser; especially better for ePubs)
• Less resource-intensive for sites (cache the generated files, then no more processing)
• Fewer problems with security plugins that disable rest api

Changes to UX

• Until pro file created (mostly just PDFs) just use browser.
• increase pdf limit to 10, 50, 100
• add new design or new section template for showing cover info at top of page

Development

• [ ] #326