Rework displaying block validation messages.

[miina]

Fixes #1679.

[kienstra]

Before, In Paired Mode (Typing Is Slow)

Before, In Classic Mode

After, In Paired Mode (Typing Is Fast Again)

Steps to reproduce:

1. Activate any theme
2. Select Paired mode
3. Checkout and build the master branch of Gutenberg (though I'm not sure the version or branch matters)
4. Create a very long post. I used the Gutenberg demo post, and copied the content 4 times so it was 5 times longer. Even then, there wasn't a huge difference, but it was noticeable.
5. Expected: typing in the block editor shouldn't be delayed
6. Actual: there's a delay between typing and seeing the text appear (see the first .gif above)

[kienstra]

Block-level Notices Still Appear

[kienstra]

Approved

Hi @miina,
Thanks a lot for fixing this issue. This PR works locally, as shown in the .gif above.

[kienstra]

Request For Review

Hi @westonruter,
Could you please review this?

Also, here's a test page on a staging site where the issue exists.

Thanks, Weston 😄

[westonruter]

Massive improvement to performance. Good work @miina!