Restructure 'file_paths' to allow for names (Closes #3320) etc. For #2765.

@coenjacobs @justinstern @maxrice These are major changes - CC ing you
guys in to take a quick look through. I know some extensions such as
vouchers will be affected.
Simpler tax rate check which Closes #3813

Could do with some more testing @ChromeOrange, and applying to .15 if
ok @coen
Merge branch 'downloadable-files-2.1'

Conflicts:
	assets/css/admin.css