Add helper needs_api_tokens()

[sebsel]

Might it be a good idea to add this helper function to helpers.php, now that there are 3 silo's that need API keys?

function needs_api_tokens($url) {
  if(Formats\GitHub::matches_host($url)) return 'github';
  if(Formats\Twitter::matches_host($url)) return 'twitter';
  if(Formats\Facebook::matches_host($url)) return 'facebook';
  return false;
}

Instead of these strings, we can also return an array of the credentials we expect.

function needs_api_tokens($url) {
  if(Formats\GitHub::matches_host($url)) return ['github_access_token'];
  if(Formats\Twitter::matches_host($url)) return ['twitter_api_key','twitter_api_secret','twitter_access_token','twitter_access_token_secret'];
  if(Formats\Facebook::matches_host($url)) return ['facebook_app_id','facebook_app_secret'];
  return [];
}