feat(chat): validate username on registration

Also, update seeds to register with usernames.

priv/repo/seeds.exs

@@ -30,7 +30,7 @@ pw = "TheFellowship"
 
 for name <- names do
   email = "#{String.downcase(name)}@fellowship.me"
-  Accounts.register_user(%{email: email, password: pw, password_confirmation: pw})
+  Accounts.register_user(%{username: name, email: email, password: pw, password_confirmation: pw})
 end
 
 aragorn = Accounts.get_user_by_email("aragorn@fellowship.me")