Migrations: delete contexts with BaseMigrator

Due to the lengthiness of this task, the migration has been adapted into
a BaseMigrator migration, running in the background instead.

priv/repo/migrations/20220807125023_data_migration_delete_context_objects.exs

@@ -3,13 +3,16 @@ defmodule Pleroma.Repo.Migrations.DataMigrationDeleteContextObjects do
 
   require Logger
 
-  @doc "This migration removes objects created exclusively for contexts, containing only an `id` field."
+  def up do
+    dt = NaiveDateTime.utc_now()
 
-  def change do
-    Logger.warn(
-      "This migration can take a very long time to execute, depending on your database size. Please be patient, Pleroma-tan is doing her best!\n"
+    execute(
+      "INSERT INTO data_migrations(name, inserted_at, updated_at) " <>
+        "VALUES ('delete_context_objects', '#{dt}', '#{dt}') ON CONFLICT DO NOTHING;"
     )
+  end
 
-    execute("DELETE FROM objects WHERE (data->>'type') IS NULL;")
+  def down do
+    execute("DELETE FROM data_migrations WHERE name = 'delete_context_objects';")
   end
 end