i18n textdomains; specifically in our JavaScript packages

[Igor]

Via the following PR I found out we forgot the translations when moving code to their own packages: #13359

That solution could be improved still. 2 things come to mind:

1. The textdomain used in the JavaScript packages are still yoast-components. Should this be changed to the package name?
2. For every package:
   1. An entry is added in the package.json scripts.
   2. A Grunt shell task is created (which runs the script above).
   3. A Grunt copy task is created (which moves the output of above).
   4. The above tasks are added to the Grunt build:i18n:potFiles alias list.

[Igor]

Closing this in favor of a comment in the readme of the javascript repo.