Fix translations in block editor.

* Fix notice appearing on Widgets Block Editor.

* Update translation text domain to `wp-revisions-control`, conforming to WordPress standards.
* Add Polish translation thanks to Maciej Gryniuk (https://github.com/odie2).

Introduces block-editor (Gutenberg) sidebar component for supported post types.

* Add bulk actions to purge excess or all revisions.
* Introduce unit tests.
* Conform to coding standards.