Add a link to skip to the main content

While people using screen readers already have keyboard shortcuts to
jump to the <main> tag, there are people who navigate the page with the
keyboard using just the tab key, and for them, this link provides a way
to save time and start reading the main content instead of having to
manually go through all the navigation links every time a new page is
loaded.

Note that we had to add an additional `width: 0` rule because
Foundation's `element-invisible` would apply `1px` and the test checking
for `visible: :hidden` would faile.

spec/rails_helper.rb

@@ -62,6 +62,8 @@ module Capybara
 
       unless url.match?("robots.txt") || url.match?("active_storage/representations")
         expect(page).to have_css "main", count: 1
+        expect(page).to have_css "#main", count: 1
+        expect(page).to have_css "main#main"
       end
     end
   end