Allow update safe modifications by custom.css and custom.js

I would suggest to integrate custom.css and custom.js by default in the core and load them after the core css and js files. These files should not be touched by updates/upgrades, so we can make update safe customizations and overrides.

There are already few hooks you can use there are head.html title.html and base.html, you can place them in your etc folder and add a location redirect to use your custom one and add your css and js there.