Dashicons: Prefix icon name to avoid bleed

[jasmussen]

Right now SVGs in the dashicons sprite have these CSS classes:

<svg class="dashicon button" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><title>Button</title><path d="M17 5H3c-1.1 0-2 .9-2 2v6c0 1.1.9 2 2 2h14c1.1 0 2-.9 2-2V7c0-1.1-.9-2-2-2zm1 7c0 .6-.4 1-1 1H3c-.6 0-1-.4-1-1V7c0-.6.4-1 1-1h14c.6 0 1 .4 1 1v5z"></path></svg>

This causes CSS bleed for the button class:

We should fix the build process upstream so the class is instead dashicons-button, to avoid this bleed, and update references.