Rely on WP.org localization tooling

[kasparsd]

Fixes #1458.

See https://developer.wordpress.org/plugins/internationalization/how-to-internationalize-your-plugin/#plugins-on-wordpress-org

Since WordPress 4.6 translations now take translate.wordpress.org as priority and so plugins that are translated via translate.wordpress.org do not necessary require load_plugin_textdomain() anymore. If you don’t want to add a load_plugin_textdomain() call to your plugin you have to set the Requires at least: field in your readme.txt to 4.6 or more.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: Describe a bug fix included in this release.
• New: Describe a new feature in this release.

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.

[marcinkrzeminski]

@kasparsd I have been reviewing this to pick this up and finish. A lot has changes since this PR was created. Can you please confirm that the bottom line of this task is:

1. Add a command that will generate *.pot file via wp i18n
2. Add a command that will generate .mo file via wp i18n
3. Above commands will be run during npm run build.
4. Set the Requires at least: 4.6
5. Remove the hook and related callback function that loads 'local' translations.