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/tasks/user_test.exs

@@ -92,7 +92,7 @@ defmodule Mix.Tasks.Pleroma.UserTest do
       assert_received {:mix_shell, :info, [message]}
       assert message =~ " deleted"
 
-      refute User.get_by_nickname(user.nickname)
+      assert %{deactivated: true} = User.get_by_nickname(user.nickname)
     end
 
     test "no user to delete" do