Namespace all CSS classes

This might be going overboard, but since webmaker-nav is meant to be embedded on arbitrary pages, I wonder if it'd be useful to namespace all its classes under e.g. `wmnav-`, so they don't conflict with pages' existing styles. Just an idea.

Start by locating the webmaker-nav CSS classes and the markup that uses them. Check how broadly the classes are used, then define the namespace boundary and verify that embedded pages no longer experience class-name conflicts; the issue does not name specific files or tests.

Written by the indexing model from the issue text.