Deactivate local users on deletion instead of deleting the record

Prevents the possibility of re-registration, which allowed to read
DMs of the deleted account.

Also includes a migration that tries to find any already deleted
accounts and insert skeletons for them.

Closes pleroma/pleroma#1687

test/web/activity_pub/transmogrifier_test.exs

@@ -872,7 +872,8 @@ defmodule Pleroma.Web.ActivityPub.TransmogrifierTest do
 
     @tag capture_log: true
     test "it works for incoming user deletes" do
-      %{ap_id: ap_id} = insert(:user, ap_id: "http://mastodon.example.org/users/admin")
+      %{ap_id: ap_id} =
+        insert(:user, ap_id: "http://mastodon.example.org/users/admin", local: false)
 
       data =
         File.read!("test/fixtures/mastodon-delete-user.json")