Reusable blocks: Bad UX for non-privileged users

[noisysocks]

There are some very rough edges when using, creating and modifying reusable blocks as an author or contributor. I've summarised the issues here as it's easier to address them as one task.

Creating a reusable block as a contributor

Expected: The Add to Reusable Blocks button should be disabled.
Actual: The Add to Reusable Blocks button is enabled. Clicking it results in an error notice appearing when the API request fails.

Updating another user's reusable block as an author

Expected: The Edit button should be disabled.
Actual: The Edit button is enabled. Clicking it lets you edit the block and save changes which results in an error notice appearing when the API request fails.

Deleting another user's reusable block as an author or contributor

Expected: The Remove from Reusable Blocks button should be disabled.
Actual: The Remove from Reusable Blocks button is enabled. Clicking it results in an error notice appearing when the API request fails.