The new `--i18n-dir` directory allows the loading of an external
directory of i18n JSON files, milar to have `--static-dir`
works. New languages can be added and existing language files
can be customized this way.
This commit changes file loading behaviour so that invalid or
non-existent don't halt the execution of the app completely but
merely throw a warning and continue with the default (en) lang.
The earlier approach of loading `/api/config.js` as a script on
initial page load with the necessary variables to init the UI is
ditched. Instead, it's now `/api/config` and `/api/settings` like
all other API calls. On load of the frontend, these two resources
are fetched and the frontend is initialised.