make new product button always visible on edit product page.

I found that it will be very useful when entering products through admin side. this will allow use to quickly navigate to product page after adding new one without going back to product list

Fixes #2407
Namespace OrderPopulator reference in OrdersController so people don't run into potential issue where it would attempt to lookup a top-level OrderPopulator class
Fixed hardcoded paths to fontawesome

Fixes #2424
Allow payment method to control auto_capture?

Some gateways (ex. samurai, eway, stripe) only support purchase, not
auth/capture. This allows those gateways to override auto_capture?, and
coexist with other payment methods not using auto_capture, and also
eliminates the required configuration for those payment methods.

Fixes #2304