Fix/missing editor script translations

[petitphp]

Context

• This PR fix the issue Missing translations in Gutenberg editor #233

Summary

This PR can be summarized in the following changelog entry:

• Fixes a bug where translations where missing in the buttons and the notices in the Block Editor. Props to @petitphp.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

Reproduce the error

• Switch WordPress to a locale other than default english (en_US), use one which is almost complete such as Italian or Dutch.
• Ensure translations files for the plugin are installed,
• Edit a published page,
• In the Publish panel on the sidebar duplicate post action links are still displayed in english Copy to a new draft and Rewrite & Republish.
• Create a Rewrite & Republish copy from a published post and see the notice is not translated

Test the patch

• Switch to the branch and build
• Switch WordPress to a locale other than default english (en_US), use one which is almost complete such as Italian or Dutch.
• Edit a published page,
• In the Publish panel on the sidebar duplicate post action links are correctly translated in current locale.
• Create a Rewrite & Republish copy from a published post and see the notice is now translated.

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• The function use to load the translations for the editor is available since WordPress 5.0. A check has been added to ensure the function is only called if available to avoid breaking older WordPress install.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #233

[enricobattocchi]

CR & Acceptance test OK!
Thanks @petitphp for your contribution and sorry for the delay, we aim to release the fix soon.