WIP: Adds Github OAuth2 support

[mdarby]

• [x] Expects GITHUB_CLIENT_ID to be set
• [x] Expects GITHUB_CLIENT_SECRET to be set
• [x] Can successfully login via Github
• [x] User data is coming back from Github
• [ ] Need to scope the orgs so that only our teams get to update (#28)
• [ ] Flash messages are seemingly persistent. Why?
• [x] Update "Manage Speakers" link after #26 lands

ProTip: So dealing with OAuth development locally sucks. Not only that, it's impossible. One great way to make it work is to use a service like forwardhq.com. Basically, they port forward your local dev port out to the web and give you a static URL. You can use that URL to set up the OAuth callbacks.

[spicycode]

@mdarby You can get around the issues you describe above by setting up a mDNS responder to give the site a domain. For example, http://columbus-elixir.localhost would work and the OAuth2 responder will be able to reach it in browser. Then you just have a Prod and Development OAuth app and dev redirects to the localhost domain.