Implement IconHeadingLabel page block

[franknoirot]

There is a component with an icon, heading, and label text that is used on the About page and across the site, that can be found in src/components/_old/PageBlock.js.

• It should be implemented as an available PageBlock (src/components/PageContent/PageBlock), but give it its own file so it's easier to find.
• Also note that if several occur in a row within page content they should be wrapped by a containing element. This is not my favorite logic but once we implement it we can think of how to improve it.