Use withProduct HOC in ReviewsByProductEditor

[Aljullu]

Part of #658.

This PR:

• Updates <ReviewsByProductEditor> so it uses the withProduct() HOC introduced in Create withProduct HOC #779.
• Refactors <ReviewsByProductEditor> to a functional component.
• Adds a loading and error state to that block.

How to test the changes in this Pull Request:

1. Create a post and add a Reviews by Product block.
2. Select a product and verify everything works as usual.
3. Reload the page and verify a loading state is shown while loading product data.
4. Modify this line as follows:

-if ( ! \current_user_can( 'edit_posts' ) ) {
+if ( true ) {

5. Verify the block correctly displays the error.

[mikejolley]

@Aljullu This works, but there is some strange loading behaviour after you select a product checkbox. It triggers loading state then shows the checkboxes again/edit view.

Maybe it shouldn't load until you click 'done'?

[Aljullu]

@Aljullu This works, but there is some strange loading behaviour after you select a product checkbox. It triggers loading state then shows the checkboxes again/edit view.

Maybe it shouldn't load until you click 'done'?

Absolutely, good catch! It should be fixed in ea9fab9. It will not show the loading screen if the user is in the edit mode.