Fixes to make the gem work

We are in the process of upgrading from cap v2 to cap v3 and routinely use the original cap v2 maintenance gem/tasks.  I was thrilled when I came across your cap v3 fork, but in testing the gem, it appears to be broken.  This PR incorporates several changes we needed in order to make this work:

- Use fetch() to pull maintenance_template_path and maintenance_basename, matching v2 behavior
- Use upload! to transfer the rendered template.
- Store the template in public/system, matching v2 behavior
- Add the v2 default template for people who don't override maintenance_template_path