Move solidus_social admin interface to submenu of settings outside of the store configuration interface

[fthobe]

Right now the administration of solidus social is confusion as all stores share the same interface with now discrimination of the store applied in the table for solidus social.

@rahulsingh321 As bespoken move social to an own menu point.

[tvdeyen]

Good idea. While doing that we can also address #138

Shipping and Payment method admin is a great example on how to use the preference store in the admin.