Get Gutenberg blocks to replace Stackable and deprecate Stackable blocks

[cjyabraham]

Now that Gutenberg blocks have matured, let's deprecate our use of Stackable and use Gutenberg blocks going forward. There are problems with keeping on the latest version of Stackable and the way it is being developed don't align with our needs. We'll need to keep Stackable around for Events that currently use it.

• Update Gutenberg block styles to mimic what we are doing with Stackable blocks
• Teach Megan how to use the Gutenberg blocks
• Turn off all Stackable blocks from the block chooser that we have found a good replacement for so that they can no longer be used

Here's a quick summary of how Stackable blocks are currently used on the site:

1. Hero Container block; easily replaced with a Group block.
2. Section Header; can be replaced with a Group block.
3. Schedule-at-a-glance Container block; can be replaced by Group.
4. Feature grid on China site
5. Image box on this page and here and here and here

We should replace the blocks we only have a few instances of (4 and 5 above) and then cut out only the css we need to render the blocks we keep (1, 2 and 3) and include it conditionally on only the pages that need them. Then we can uninstall the stackable plugin entirely.

Alternatively we could go through all old pages and replace all stackable blocks.

[cjyabraham]

We're going to hold off on this for now until Megan and her team have time to absorb a change like this and also until Gutenberg matures a bit more.

[cjyabraham]

Find my blocks could be useful to find where Stackable is being used.

[cjyabraham]

Closing in favor of #576