chore: enforce unique linked_ids (#12815)

* chore: enforce unique linked_ids

Duplicate linked_ids make no sense. 2 users cannot share the same
source user from a provider

coderd/database/migrations/000205_unique_linked_id.up.sql

@@ -0,0 +1,21 @@
+-- Remove the linked_id if two user_links share the same value.
+-- This will affect the user if they attempt to change their settings on
+-- the oauth/oidc provider. However, if two users exist with the same
+-- linked_value, there is no way to determine correctly which user should
+-- be updated. Since the linked_id is empty, this value will be linked
+-- by email.
+UPDATE ONLY user_links AS out
+SET
+	linked_id =
+		CASE WHEN (
+			  -- When the count of linked_id is greater than 1, set the linked_id to empty
+			  SELECT
+			      COUNT(*)
+			  FROM
+			      user_links inn
+			  WHERE
+			      out.linked_id = inn.linked_id AND out.login_type = inn.login_type
+		  ) > 1 THEN '' ELSE out.linked_id END;
+
+-- Enforce unique linked_id constraint on non-empty linked_id
+CREATE UNIQUE INDEX user_links_linked_id_login_type_idx ON user_links USING btree (linked_id, login_type) WHERE (linked_id != '');