Merge branch 'refactor/db-not-null-constraints-for-scheduled_activities' into 'develop'

Add NOT NULL constraints for scheduled_activities table

See merge request pleroma/pleroma!1907

priv/repo/migrations/20191026191711_set_not_null_for_scheduled_activities.exs

+defmodule Pleroma.Repo.Migrations.SetNotNullForScheduledActivities do
+  use Ecto.Migration
+
+  # modify/3 function will require index recreation, so using execute/1 instead
+
+  def up do
+    execute("ALTER TABLE scheduled_activities
+    ALTER COLUMN user_id SET NOT NULL")
+  end
+
+  def down do
+    execute("ALTER TABLE scheduled_activities
+    ALTER COLUMN user_id DROP NOT NULL")
+  end
+end