Describe the bug

When creating a reusable block, the "Block created" confirmation message appears but remains persistent until the user actively dismisses it.

The same occurs when updating (for example, naming) a reusable block. The "Block updated" message is persistent until the user dismisses it.

This results in confirmation message stacking when saving the post. See screenshot.

To Reproduce
Steps to reproduce the behavior:

1. Create a new post.
2. Insert content into a block
3. Convert the block into a reusable block
4. Note the confirmation message, and note that it is persistent
5. Publish the post
6. Note the "Post published" message and the "Block created" message stack on top of each other

Expected behavior

Ideally the confirmation messages for reusable blocks would disappear after a timeout, or disappear at least when the post is published to avoid stacking.

Screenshots

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Version: 70

Additional context

• Gutenberg version: WordPress 5.0 beta 5