[xl] replace auth0 with simple login

[subhankarb]

This commit replaces auth0 login with github login. Changes made are listed below

• Update API /api/auth/login
• Update API /api/auth/callback
• Removed AUTH0 model from auth.models
• Added Flask-OAuthlib in dependency.
• Update profile.models.User (added oauth_source, removed auth0_id, added unique constrain on email)
• Remove lock from base.html and index.html, and replace lock method call to /api/auth/login API
• Update configuration. Added github configuration and remove auth0
• Remove AUTH0 config from README and added github config
• Update relevant test cases.

Resolves: #268 See also: #267

[rufuspollock]

@subhankarb LGTM and excellent commit message.