When the modal is shown, then we set the "src" attribute
of the iframe, which causes the browser to load the content.
We only do this the first time the modal is shown, because
the src will be the same for any given page. Fixes GH #1179.

It includes the release name and a button to close the modal.

See twbs/bootstrap#4854

Displaying the dependency graph modal isn't really
a "navagation" event.  So it is more appropriate to
implement this as a <button> rather than an <a> link.

However, Bootstrap has slightly different styling for
buttons, so I had to add a bit of CSS to make this particular
button look just like the neighboring links.

I also moved the modal <div> outside the <ul> since it wasn't
strictly valid, and it makes the markup easier to understand
when/if you need to look at the page source.

Delay loading of dependency graph until it is shown.