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_Update.sql

@@ -0,0 +1,20 @@
+CREATE PROCEDURE [dbo].[SsoUser_Update]
+    @Id BIGINT OUTPUT,
+    @UserId UNIQUEIDENTIFIER,
+    @OrganizationId UNIQUEIDENTIFIER,
+    @ExternalId NVARCHAR(50),
+    @CreationDate DATETIME2(7)
+AS
+BEGIN
+    SET NOCOUNT ON
+
+    UPDATE
+        [dbo].[SsoUser]
+    SET
+        [UserId] = @UserId,
+        [OrganizationId] = @OrganizationId,
+        [ExternalId] = @ExternalId,
+        [CreationDate] = @CreationDate
+    WHERE
+        [Id] = @Id
+END