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.

frontend/angular.json

@@ -170,6 +170,10 @@
           },
           "configurations": {
             "production": {
+              "assets": [
+                "src/favicon.ico",
+                "src/robots.txt"
+              ],
               "fileReplacements": [
                 {
                   "replace": "src/environments/environment.ts",