User, Migration: Change `accepts_chat_messages` to be nullable

This is to model the ambiguous state of most users.

lib/pleroma/user.ex

@@ -138,7 +138,7 @@ defmodule Pleroma.User do
     field(:also_known_as, {:array, :string}, default: [])
     field(:inbox, :string)
     field(:shared_inbox, :string)
-    field(:accepts_chat_messages, :boolean, default: false)
+    field(:accepts_chat_messages, :boolean, default: nil)
 
     embeds_one(
       :notification_settings,

priv/repo/migrations/20200703101031_add_chat_acceptance_to_users.exs

@@ -1,12 +1,17 @@
 defmodule Pleroma.Repo.Migrations.AddChatAcceptanceToUsers do
   use Ecto.Migration
 
-  def change do
+  def up do
     alter table(:users) do
-      add(:accepts_chat_messages, :boolean, nullable: false, default: false)
+      add(:accepts_chat_messages, :boolean, nullable: true)
     end
 
-    # Looks stupid but makes the update much faster
+    execute("update users set accepts_chat_messages = true where local = true")
-    execute("update users set accepts_chat_messages = local where local = true")
+  end
+
+  def down do
+    alter table(:users) do
+      remove(:accepts_chat_messages)
+    end
   end
 end