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.

app/components/layout/skip_to_main_content_component.rb

@@ -0,0 +1,2 @@
+class Layout::SkipToMainContentComponent < ApplicationComponent
+end