[WIP] Changed core icons from <img> to <div> + CSS

Just looking for feedback/thoughts on this.  So instead of rendering Jenkins UI icons using a raw &lt;img> tags, we render using a &lt;div> + CSS.  Note we are not proposing to change the actual icons with this change i.e. the user still sees the same icons.

The <div> + CSS approach allows a lot more control over the icons.

So in these changes in core Jenkins, you'll see in the Jelly scripts that instead of rendering the icons like:

```
<img src="${imagesURL}/48x48/blue.png" alt="" height="48" width="48"/>
```

You'd use a new <l:icon> Jelly tag which just names the icon you want placed in that location:

```
<l:icon name="blue-48x48"/>
```

The core icons are all then rendered using CSS: https://github.com/tfennelly/jenkins/blob/icons-via-css/war/src/main/webapp/css/icons.css

If we go this route, we'll provide backward compatibility for plugins that will allow the plugin to continue working on older versions of Jenkins.

Wdyt?
Ah nice one... thanks Robert.