Sso user table, model and repo stubbed out (#837)

* Sso user table, model and repo stubbed out

* switch to nullable org id, bigint id

* update GetBySsoUserAsync

* cleanup migrator file

* fix EF user repo

* fix pg repo

* is `IS NULL` checks

* unique indexes

* update migration scripts

* add another unique index

* remove old script

src/Sql/dbo/Stored Procedures/SsoUser_Delete.sql

@@ -0,0 +1,14 @@
+CREATE PROCEDURE [dbo].[SsoUser_Delete]
+    @UserId UNIQUEIDENTIFIER,
+    @OrganizationId UNIQUEIDENTIFIER
+AS
+BEGIN
+    SET NOCOUNT ON
+
+    DELETE
+    FROM
+        [dbo].[SsoUser]
+    WHERE
+        [UserId] = @UserId
+        AND [OrganizationId] = @OrganizationId
+END