Change CAPTCHA handling to be only on email verification

This simplifies the implementation considerably, and while not providing
ideal UX, it's the most flexible approach.

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

@@ -10,7 +10,6 @@ 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(:render_captcha_if_needed).and_return(nil)
   end
 
   it 'has valid open graph tags' do