Refactor external link handling

[caleywoods]

On the standards website we use the USWDS usa-link--external class to render an icon to denote that the link in question leaves the .gov top level domain. This class is applied through a small amount of client-side JavaScript once the DOM content is loaded. This approach was originally used for developer speed but a better approach is to move this code into the markdown rendering process so that there's no client-side JS involved which will save a bit a few bytes on the wire.

The markdown parser in use is called markdown-it which implements the CommonMark spec and there are a few times during the rendering process that we need to supply code to make sure that we get all links checked and the class applied. These scenarios/areas are:

• The link_open rendering step which happens when the renderer encounters the beginning an anchor element in the token stream. This code alone gets most links but there's a couple of cases it doesn't handle. [18F site example code]
• The next scenario happens when the renderer encounters an HTML block inside of a markdown document. The renderer rule for this is named html_block. [18F site example code]
• The final scenario happens when the renderer encounters some inline HTML which is a rule named html_inline.[18F site example code]