Made WebAdmin more modern

I've made the webadmin look more modern using the google designing standards.

* Light/Bold text for captions and important stuff
* .warn, .info, .err css class for error messages
* a bit more margin and padding and increased font size for improved readability
* made the website responsive until a width of about 650px
* made it on larger screens wider for more informations. Long texts will be shorter in line height
* made button and input styles to match everything
* added page specific css for some core plugins
* page specific css can be easily turned off
After open sans, you should specify `sans-serif` to allow fallback to a reasonable font if the user has no internet connection.