Move admin as a prefix out of the plugin

[m0sviatoslav]

Move route name admin as a prefix out of the plugin.

The way it's done in AdminBundle

sylius_admin:
    resource: "@SyliusAdminBundle/Resources/config/routing.yml"
    prefix: /admin

The way it's done in the plugin
now:

sylius_admin_order_creation:
    resource: "@SyliusAdminOrderCreationPlugin/Resources/config/app/routing.yml"

change to:

sylius_admin_order_creation:
    resource: "@SyliusAdminOrderCreationPlugin/Resources/config/app/routing.yml"
    prefix: /admin

and remove admin from SyliusAdminOrderCreationPlugin/Resources/config/app/routing.yml

if it's okay I can commit changes

[m0sviatoslav]

#118