Add 'No saved blocks' message to Inserter

[noisysocks]

Fixes #3994.

Previously, a brand new user would encounter a completely blank tab when selecting the Saved tab in the inserter.

I've fixed this by adding a simple message that indicates that there are no saved blocks:

Before	After

I also removed the category headers from the Embed and Saved tabs, which I think looks cleaner:

Before	After

I assume that this was the intended design based on the presence of an if ( 'embed' === tab ) check that was never executing (because of a typo: it should be embeds).

How to test

1. Ensure you have no reusable blocks. You could do this by testing on a new site, running wp site empty, or executing a DELETE FROM wp_posts WHERE post_type = 'wp_block'.
2. Open the inserter and click on Saved — you should see the message.
3. Check that other parts of the inserter work correctly.

[noisysocks]

cc. @jasmussen @karmatosed @youknowriad

[jasmussen]

Looks good visually! 👍

[youknowriad]

LGTM 👍

[aduth]

There's redundancy / inconsistency between this and #4040. Do we really need a custom message to explain that there's no saved blocks, or could we simplify to a standard "No blocks found" for all of the following cases:

• There are truly no blocks registered
• There are no results for a given search
• There are no saved blocks

[noisysocks]

I think specific copy provides a nicer user experience, but agree that those messages should all use the same component and appear visually similar.

[jasmussen]

Though I agree a customized message is a nicer user experience, I would agree with aduth that it would be nice to simplify as much as we can, and if we can simplify the code by reusing an existing message, I think we should do that.

[karmatosed]

I would +1 for simplifying. If we have time adding in customized messages in a lot of places would be amazing, but for now lets go with simple implementation.