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.
Fixes test error:
```
>> Problem: [http_splitting] Possible HTTP-Splitting vulnerability.
Description: Using variables that can contain "\n" or "\r" may lead to http injection.
```
Summary: `$uri` should never be used in `return` statements.
See: https://github.com/yandex/gixy/blob/master/docs/en/plugins/httpsplitting.md
In this case, `$uri` always equals `/`, so just replace it.
