Make the default server address `0.0.0.0` to make the server accessible
from inside a Docker container.

When adding a GitHub commit status, instead of using the job "home" URL,
use the URL of its "Console" page. This page shows the build log and is
much more useful than the other one. If there's a build failure in
particular, you'd be going to this page anyway to see what the problem
was, so using this `target_url` saves a click.

A new function that will return the URL to the Jenkins "Console" page
given the URL of a Jenkins job.

We'll be using this to pass with the GitHub status request as the
`target_url`.

There are a bunch of events that can trigger the `pull_request` webhook.
These are:

    "assigned", "unassigned", "review_requested",
    "review_request_removed", "labeled", "unlabeled", "opened",
    "edited", "closed", or "reopened". And "synchronize".

We only care about "opened" and "synchronize" because those are the only
two where new commits can come through. Since we don't set commit
statuses unless that commit is part of a pull request, we need a way to
update the status the first time a pull request is opened, thus the
"opened" action. When new commits are added we also want to update
statuses, so "synchronize". But for the others, we don't want to be
duplicating work and adding duplicate unnecessary statuses to PR
commits.