Change local unique username constraints.

server/migrations/2020-04-21-123957_remove_unique_user_constraints/down.sql

@@ -0,0 +1,4 @@
+-- The username index
+drop index idx_user_name_lower_actor_id;
+create unique index idx_user_name_lower on user_ (lower(name));
+

server/migrations/2020-04-21-123957_remove_unique_user_constraints/up.sql

@@ -0,0 +1,2 @@
+drop index idx_user_name_lower;
+create unique index idx_user_name_lower_actor_id on user_ (lower(name), lower(actor_id));