Since 355e89ce5, the frontend references resources via root-relative URLs.
This means that `resources` dirs in the language dirs are no longer
accessed and can be removed.
Achieve this by defining a specific `assets` production config that
doesn't include `src/resources`.
As of fd35c8f4a, this shrinks the frontend size by 55% (279M -> 124M).
Also, the nginx location configs now can be simplified.
22def9b01c