fix: add postgres triggers to remove deleted users from user_links (#12117)

* chore: add database test fixture to insert non-unique linked_ids
* chore: create unit test to exercise failed email change bug
* fix: add postgres triggers to keep user_links clear of deleted users
* Add migrations to prevent deleted users with links
* Force soft delete of users, do not allow un-delete

coderd/database/queries/users.sql

@@ -116,7 +116,7 @@ WHERE
 UPDATE
 	users
 SET
-	deleted = $2
+	deleted = true
 WHERE
 	id = $1;