title: Get referring URL for Flask request
body: <p>When a user visits our site and signs up, how can I capture which website they came from?</p>
  
  <p>Be it search, a PR website, etc. I don't care what page from our site they visited, I just want to know which marketing efforts are giving us the most signups.</p>
  
  <p>I know Google Analytics can probably do this but I'd like to have something internal for reference as well.</p>