Moving some background jobs into simple tasks

- fetching activity data
- attachment prefetching
- using limiter to prevent overload

priv/repo/migrations/20200915095704_remove_background_jobs.exs

@@ -0,0 +1,22 @@
+defmodule Pleroma.Repo.Migrations.RemoveBackgroundJobs do
+  use Ecto.Migration
+
+  import Ecto.Query, only: [from: 2]
+
+  def up do
+    from(j in "oban_jobs",
+      where:
+        j.queue == ^"background" and
+          fragment("?->>'op'", j.args) in ^[
+            "fetch_data_for_activity",
+            "media_proxy_prefetch",
+            "media_proxy_preload"
+          ] and
+          j.worker == ^"Pleroma.Workers.BackgroundWorker",
+      select: [:id]
+    )
+    |> Pleroma.Repo.delete_all()
+  end
+
+  def down, do: :ok
+end