Accept reason in POST /api/v1/accounts and store in DB

priv/repo/migrations/20200714043918_add_registration_reason_to_users.exs

@@ -0,0 +1,9 @@
+defmodule Pleroma.Repo.Migrations.AddRegistrationReasonToUsers do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      add(:registration_reason, :string)
+    end
+  end
+end