Restoration of solidus_social and implementation for starter frontend

[fthobe]

This issue contains all tasks to restore / bugfix solidus_social for the current maintained version of solidus (4.4.2).

Blocking Issues

• Improve visual experience on starter_frontend (see starter_frontend_issue 404
• Resolve deprecation messages
• Twitter Login OAuth::Unauthorized (400 Bad Request) #85
• Solidus v4 compatibility? #114
• ::Spree::UserRegistrationsController.prepend self - not found on install #115
• Align icons with starter frontend #125
• Admin Interface Broken #124

Nice to have

• Merge guest user cart with authenticated one #81
• Auth providers are not related to stores #128

[fthobe]

@mamhoff do you have any wishes regarding additional default options, I saw you put some touches on this gem.

[fthobe]

@jarednorman all tickets checked above can be closed