Redesign /about when already logged in (#17348)

* Redesign /about when already logged in

* Fix sign up form still showing when OMNIAUTH_ONLY is set

* Fix tests

* Change wording based on suggestions

Co-authored-by: Eugen Rochko <eugen@zeonfederated.com>

Co-authored-by: Eugen Rochko <eugen@zeonfederated.com>

spec/views/about/show.html.haml_spec.rb

@@ -8,6 +8,7 @@ describe 'about/show.html.haml', without_verify_partial_doubles: true do
     allow(view).to receive(:site_title).and_return('example site')
     allow(view).to receive(:new_user).and_return(User.new)
     allow(view).to receive(:use_seamless_external_login?).and_return(false)
+    allow(view).to receive(:current_account).and_return(nil)
   end
 
   it 'has valid open graph tags' do