Fixes #15037

The js and php code of n() has fallbacks for when the key is missing.
However there is no fallback, when the key is defined with an array of empty
strings. So we just don't extract them anymore and use the english language.

Ported from the administration repo:
https://github.com/owncloud/administration/commit/c67eaa11c04d0989ee9331c97788a0225a82e7f5
and
https://github.com/owncloud/administration/commit/41b0f9fa3a9c6183972f971eea3875b98e22217a

adding console command to generate javascript translation files based on existing php translation files
read server side translations from json files