Use Deep Linking for logging in using native app on mobile devices

[amiller]

We can tell from the user agent fields in the request headers if the requester is on a mobile or a desktop.

Our current redirect to https://twitter.com/oauth/oauth_verify works fine for desktop but on mobile it's inconvenient since its a lot more likely that users are logged in via native app but not the browser.

• check whther user agent is mobile or desktop
• if mobile, redirect to the deeplink like
  twitter://oauth/authenticate?oauth_token={}

[amiller]

It seems more complicated because the redirect from the twitter app doesn't return back to the browser