Apply custom stylesheets using a setting

In order to apply custom stylesheets the current requirement is to override one or more templates to make sure the desired files get imported. This is probably fine in many cases but can be seen as an unnecessary complication when the only changes someone wants to make to the dashboard is a few style changes. For example, a company wants to apply some branding styles to the dashboard. This can be simplified by adding a setting that allows specifying the location of custom stylesheet files. Instead of having to mess with overriding templates they just need to add their branding stylesheet to the list of custom styles to import.

This would be very similar to the current angular_modules and js_files settings under HORIZON_CONFIG.