Language generation and file structure

* Move language files to strings/ subdir to be consistent with
   react-sdk
 * Only copy static list of languages (to avoid including
   languages that are only a few percent translated)
 * Make copy-res script work with watch mode
 * Other general cleanups like only write each language file once,
   rather than n times.

src/i18n/strings/basefile.json

@@ -0,0 +1 @@
+{}