Add a default size to SVGs

[ComedyTomedy]

One problem with SVG icons is that if the page loads before CSS you get a flash of a gigantic unstyled icon. To quote the new-ish design system of the W3C website:

Always include width and height attributes as part of the SVG markup. This prevents the SVG from expanding to fill the whole page if the CSS doesn't load. - https://design-system.w3.org/styles/svg-icons.html

The fix is to add the following attribs to the inline svg element:

width="1em" height="1em"

Values assigned with CSS will always override these defaults, so it won't be a breaking change (unless someone wants page-sized icons)

[ComedyTomedy]

Just looked at the code and discovered this might not be so easy!

Perhaps better done upstream, or presumably it would be easy to patch them into the hiccup version in svg->electric, but feels a little unclean.