Separate core php-debugbar assets from customisations

[robbieaverill]

To make way for pre-processed assets (e.g. #53).

Currently the assets/debugbar.css for example has some customisations. We should use a location that is obvious that it's a third party file that shouldn't be touched.

Acceptance criteria

• Latest version of php-debugbar's assets are copied through to assets/vendor/php-debugbar
• Customisations to assets/debugbar.css as well as the other CSS and Javascript files in that folder are moved to a new folder (e.g. css/)
• Documentation makes it easy for devs to find out which files to change when contributing

[lekoala]

I've made some progress on this with https://github.com/lekoala/silverstripe-debugbar/commit/b0831765a71765045f7eb3f81f28ddf846f34987 but i'm not sure how to proceed for js files