Provide a permission to admin reusable blocks.

Ideally, users should be able to use reusable blocks, just don't have permissions to edit them.

This is exactly what this patch is doing, they users without the permission are able to see the "Reusable" tab and add any of the existing reusable blocks, but they are not able to create new ones or edit the existing ones, so it is possible to control who manages reusable blocks.

And the reason it is being converted to regular block automatically is to improve the UX because as mentioned in the comment #4, it is really common that new Gutenberg users don't know they are editing a reusable block, so they change it accidentally.

Rerolled and moved to needs review.

great work @jidrone, it appears you've made numerous improvements, fixes and cleanup here in addition to addressing reusable block challenges.

We've also come accross this issue as mentioned in comment #4

I would also appreciate the addition of these permissions so I can prevent editors from using the reusable blocks. There have been inadvertant updates since the editor did not realize it updated the block on all pages.