New CTA partials

Have moved Basic and Pro CTA's from inline HTML to global variables. This means button copy for 'start taking payments' and 'contact sales' can be updated throughout the site in one place (`metadata.js`)

Aim is to help maintain consistent CTAs globally, and make it easier for marketing to update as and when necessary.

Lots of files changed here, quick review @jamesshedden 

FAO: @angusbayley 
This feels wrong - if Marketing change the CTA_BASIC this sentence might stop making sense. Shall we put it back to text in this location?