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/sync-assets.js

@@ -4,7 +4,7 @@ var fs = require('fs');
 const CONFIG_FILE_NAME = 'mempool-frontend-config.json';
 let configContent = {};
 
-var PATH = 'dist/mempool/browser/en-US/resources/';
+var PATH = 'dist/mempool/browser/resources/';
 if (process.argv[2] && process.argv[2] === 'dev') {
   PATH = 'src/resources/';
 }