favicon doesn't show up on the Superuser's admin

[lalver1]

          Maybe need a future ticket: The favicon doesn't show up on the superuser homepage, but it does for the other admin pages: log in, log out, in-person flow pages. 

Originally posted by @machikoyasuda in #2430 (comment)

The favicon is not showing up in the Superuser's admin page, but it seems like we can use the extrahead block of the admin app's base.html template to include it. So we may need to override the admin/index.html template and include the icon in the extrahead block.