Check if `image` is defined on product image partial

Rails no longer generates local vars for partials without passing them
explicitly. See https://github.com/rails/rails/commit/c67005f
Replace PUT for PATCH

As PATCH is the new default HTTP method for updates in Rails 4. At this
point frontend build looks all green \o\o/\o/\o/
Fix backend / dash admin_path conflict

The named route `admin` has been renamed to `admin_overview` on the dash
gem and the the admin_path put back into backend gem. Because the
backend gem depends heavily on the `admin_path` helper and I don't think
it should depend on the dash gem I figured it just safer to rename the
named route on the dash gem instead.

See https://github.com/rails/rails/pull/9704 for details on why (?!) the
app now breaks when the same named route is defined twice
Add params `permit` to backend controllers

backend specs pass except the ones that make assertions against
page content and do not use js driver. Navigating through backend and
creating / updating records should be fine at this point though
Remove `spree.root_path` reference from backend

Since root_path is no longer defined on the backend gem

[Fixes #3157]