Implements IExternalLoginService and the db table, implements more logic to start enabling this to work in the back office, need to implement the rest of the userstore and then implement a way once logged in to the back office to allow users to link their accounts with external logins. Currently if an external login is detected during startup and it has not been linked we'll throw an exception. Very very close to making this all work nicely.

src/Umbraco.Core/Constants-Web.cs

@@ -22,6 +22,8 @@
 	    public static class Security
 	    {
 
+            public const string BackOfficeAuthenticationType = "UmbracoBackOffice";
+
 	        public const string StartContentNodeIdClaimType = "http://umbraco.org/2015/02/identity/claims/backoffice/startcontentnode";
             public const string StartMediaNodeIdClaimType = "http://umbraco.org/2015/02/identity/claims/backoffice/startmedianode";
             public const string AllowedApplicationsClaimType = "http://umbraco.org/2015/02/identity/claims/backoffice/allowedapps";