Change webpush endpoint to unlimited varchar

Fixes #552

priv/repo/migrations/20190126160540_change_push_subscriptions_varchar.exs

@@ -0,0 +1,9 @@
+defmodule Pleroma.Repo.Migrations.ChangePushSubscriptionsVarchar do
+  use Ecto.Migration
+
+  def change do
+    alter table(:push_subscriptions) do
+      modify(:endpoint, :varchar)
+    end
+  end
+end