Require user names to be unique after unicode normalisation

As with the previous checks on case sensitivity this only affects
new users, and changes to names of existing users.

db/migrate/20231213182102_add_canonical_user_index.rb

@@ -0,0 +1,7 @@
+class AddCanonicalUserIndex < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    add_index :users, "LOWER(NORMALIZE(display_name, NFKC))", :name => "users_display_name_canonical_idx", :algorithm => :concurrently
+  end
+end