Zero down-time deploy and server checks

This change makes Dokku start up the new container, run a set of checks
against it, and only switch traffic over to the new containers if all
checks complete successfully. No requests are dropped during the switch
over.

To specify checks, add a `CHECKS` file to the root of your project
directory. This is a text file with one line per check. Empty lines and
lines starting with # are ignored.

A check is a relative URL and may be followed by expected content from
the page, for example:

```
/about     Our Amazing Team
```

Even if you don’t use any checks, this change will prevent downtime
during switching from old to new container.

Thanks to https://github.com/michaelshobbs for simplifying the process by grabbing checks from the project directory.

See: https://labnotes.org/zero-downtime-deploy-with-dokku/