ZPM UI overhaul

Fixes https://github.com/zerovm/zpm/issues/109.
Before, the `index.html.tmpl` file was run through Jinja2 twice: once when bundling and once when deploying (to finally generate an `index.html` file).

Now I see that it is only rendered when deploying and that you pass in the `zapp` at that time. That sounds much better.