Update Donate site's navigation bar

[data-sync-user]

We have updated the foundation site to use the new, user-curated navigation bar. However, we found that pages such as foundation.mozilla.org/donate are still using the old navigation bar.

The design team has requested that we update the donate site’s pages to use the new navigation bar as well. Since no other pages will be using the old nav bar, we can also remove the FoundationNavigationMixIn from the codebase!

┆Issue is synchronized with this Jira Task