Reserve a few user names

These are all names that are used for domain.com/:route routes or projected to be.

lib/pleroma/user.ex

@@ -197,6 +197,7 @@ defmodule Pleroma.User do
       |> validate_confirmation(:password)
       |> unique_constraint(:email)
       |> unique_constraint(:nickname)
+      |> validate_exclusion(:nickname, Pleroma.Config.get([Pleroma.User, :restricted_nicknames]))
       |> validate_format(:nickname, local_nickname_regex())
       |> validate_format(:email, @email_regex)
       |> validate_length(:bio, max: 1000)