text: Title: "Add to Slack" Button throws OAuth error
  Body: The "Add to Slack" button here https://api.slack.com/docs/slack-button fails with the error
  
  Oops, Something Went Wrong!
  Please try again from here or contact the app owner (reason: invalid_browser: This can occur due to page reload, not beginning the OAuth flow from the valid starting URL, or the /slack/install URL not using https://)
  
  This is repeatable on Firefox & Chrome.
  
  The https://XXX.com/slack/install button works. I can also get the "Add to Slack" button to temporarily work if I first go to the https://XXX.com/slack/install page even if I do not click anything there.
  
  Is there a function that https://XXX.com/slack/install is calling when it loads or a setting I am missing to get this to work?
  
  I am using the URL https://slack.com/oauth/v2/authorize?client_id={{ client_id }}&scope={{ scopes }}&state={{ unique_user_code }} for the button 