Gutenberg: fix box-sizing for icons and drop down menu

[gwwar]

Changes proposed in this Pull Request

This PR adds missing box-sizing style to fix centering and sizing of buttons.

After

Before

Testing instructions

• Checkout this branch
• visit /gutenberg
• select a site
• start typing and look at block options as shown in the images above.
  Expected: we don't see the visual defects shown in the "Before" picture

Fixes #27361

[matticbot]

Test live: https://calypso.live/?branch=update/button-styling

[ghost]

That's a great PR description, thank you so much for your effort!

Generated by 🚫 dangerJS

[Copons]

Just wanted to say that, apparently, lots of the glitches we're having with Gutenberg in Calypso are caused by the different box-sizing.
Gutenberg defaults everything to border-box, while Calypso doesn't make such assumption and leaves the freedom to define it as needed.

I've realized it at some point during last week, but I've failed to write about it. It would likely simplify the styling fixes, and I'd love to do a bit of a refactor on the fixes I've already merged.