Add rm_user mix task

lib/mix/tasks/rm_user.ex

+defmodule Mix.Tasks.RmUser do
+  use Mix.Task
+  import Mix.Ecto
+  alias Pleroma.{Repo, User}
+
+  @shortdoc "Permanently delete a user"
+  def run([nickname]) do
+    ensure_started(Repo, [])
+
+    with %User{local: true} = user <- User.get_by_nickname(nickname) do
+      Repo.delete!(user)
+    end
+  end
+end