[Saved objects] Prevent UI export of managed SOs

[drewdaemon]

This issue does not include a change to the default behavior of the SO export API

After a discussion with @pgayvallet and @stratoula we've decided to go with preventing managed content from being exported through the SO management view (API behavior will remain unchanged) with an exception for managed data views.

We don't see a compelling use case for exporting managed content outside of the common case where users build content that depends on the main Fleet data views (logs-*, metrics-*).

One drawback is that allowing users to export managed data views opens the door to overwriting Fleet's installed data views during an import. We think this is okay because

• managed data views won't be marked read-only as part of managed content
• the SO import API already gives them leeway to overwrite content

If this turns out to annoy users, we can revisit #172392