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

priv/repo/migrations/20200428221338_insert_skeletons_for_deleted_users.exs

@@ -0,0 +1,45 @@
+defmodule Pleroma.Repo.Migrations.InsertSkeletonsForDeletedUsers do
+  use Ecto.Migration
+
+  alias Pleroma.User
+  alias Pleroma.Repo
+
+  import Ecto.Query
+
+  def change do
+    Application.ensure_all_started(:flake_id)
+
+    local_ap_id =
+      User.Query.build(%{local: true})
+      |> select([u], u.ap_id)
+      |> limit(1)
+      |> Repo.one()
+
+    unless local_ap_id == nil do
+      # Hack to get instance base url because getting it from Phoenix
+      # would require starting the whole application
+      instance_uri =
+        local_ap_id
+        |> URI.parse()
+        |> Map.put(:query, nil)
+        |> Map.put(:path, nil)
+        |> URI.to_string()
+
+      {:ok, %{rows: ap_ids}} =
+        Ecto.Adapters.SQL.query(
+          Repo,
+          "select distinct unnest(nonexistent_locals.recipients) from activities, lateral (select array_agg(recipient) as recipients from unnest(activities.recipients) as recipient where recipient similar to '#{
+            instance_uri
+          }/users/[A-Za-z0-9]*' and not(recipient in (select ap_id from users where local = true))) nonexistent_locals;",
+          [],
+          timeout: :infinity
+        )
+
+      ap_ids
+      |> Enum.each(fn [ap_id] ->
+        Ecto.Changeset.change(%User{}, deactivated: true, ap_id: ap_id)
+        |> Repo.insert()
+      end)
+    end
+  end
+end