Explore new implementation for link weights

[Scotchester]

In cfpb/design-manual#262, @jimmynotjim wondered if there might be a more efficient way to set link weights:

now that Avenir is the default, could we globally set all links to Avenir Medium, and use [parent selectors for areas where we only expect inline links to set them to Regular]?

(Edited to reflect the fact that Georgia is no longer in the picture.)

Let's explore this idea.