Fix relative paths. FC layer for Symfony to emulate Twig asset()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| Refs tickets  | 
| License       | MIT
| Doc PR        | 

The problem is our themes mostly use relative paths. When using routed requests like `/demo` then all URLs are written from `/demo/...` so are wrong. This is solved in Symfony because templates use `asset()`. Without this patch there is no way to mix Symfony and smarty based themes.