[UX] Add an admin preview of the type of menu for menu blocks in the "Manage blocks" page.

I thought of this while creating a PR for #2324:

I thought that it would be great if there was a "Rendered as: [menu-type]" text added to the blocks in the "Manage blocks" page. That way people could get an overview of whether the menu block is set to drop-down, top-level only or tree without having to either open the block configuration dialog or visit the front-end to actually see how it is rendered for the specific layout they are editing (similar to the block visibility conditions summary), ... It would have to be in the block description on the "Manage blocks" page only and not on the descriptions that are shown in the "Add block" dialog. Perhaps I'll keep exploring that in a separate issue.

...well, this is the issue for it