Add yoast warning component

Summary

This PR can be summarized in the following changelog entry:

Adds an exclamation triangle to the SvgIcon component.
Adds a YoastWarning component that consists of an exclamation triangle followed by a message on a yellow background.

This PR can be tested by following these steps:

Fetch remote branches: git fetch --all.
Switch to this branch: git checkout 11700-add-yoast-warning-component.
Install dependencies: yarn.
Run the stand-alone: yarn start.
Go to this url in your browser: http://localhost:3333/.
Go to the Components page (top right).
Verify that the Yoast warning is there and looks like this:
Switch the language direction.
Verify that the Yoast warning looks like the following. Watch the margins between the icon and the message.