[WebProfilerBundle] Use inline images instead of asset() in form-panel

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | silexphp/Silex-WebProfiler#33
| License       | MIT
| Doc PR        | 

In the ```form.html.twig```-template in dev-master the twig function ```assert()``` is used to load the images for the toggle-buttons. This is bad, as the profiler is also used in other projects, which maybe do not have an ```assert()``` function, like Silex.
 In version 2.4.x this is not an issue, as the new images were introduced for 2.5.

I change the assert() part to inline data-images with this [converter](http://www.askapache.com/online-tools/base64-image-converter/). Compression was enabled, so the images should be as small as possible. I also compressed the toolbar-icon a little bit further.

While switching between before (dev-master) and after (this PR) I have not spotted any visual difference.