Allow users with Editor role to create, edit quotes

At present users need an Administrator role (or the Admin-specific manage_options capability) in order to create and edit invoices fully. Without that capability the plugin does not allow users to find clients in the Quote / Invoice pages.

Giving users an Administrator role when they do not need Administrator functionality can create security issues in some contexts. So it would be useful if the plugin allowed users with the Editor role to find clients in order to manage the quotation / invoice process.