frontend: Don't copy resources to language dirs

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.

production/nginx/server-common.conf

@@ -58,12 +58,6 @@ location = / {
 	expires 5m;
 }
 
-# used to rewrite resources from /<lang>/ to /en-US/
-# cache /resources/** for 1 week since they don't change often
-location ~ ^/[a-z][a-z]/resources/(.*) {
-	try_files $uri /en-US/resources/$1 =404;
-	expires 1w;
-}
 # cache /<lang>/main.f40e91d908a068a2.js forever since they never change
 location ~ ^/([a-z][a-z])/(.+\..+\.(js|css)) {
 	try_files $uri =404;
@@ -84,7 +78,7 @@ location ~ ^/([a-z][a-z])/ {
 
 # cache /resources/** for 1 week since they don't change often
 location /resources {
-	try_files $uri /en-US/$uri /en-US/index.html;
+	try_files $uri /en-US/index.html;
 	expires 1w;
 }
 # cache /main.f40e91d908a068a2.js forever since they never change