issue 451 - MakeDrive auth should be pluggable, configurable, optional

I think we need to abstract this a bit more.  In order to use custom authentication strategies, you have to load other modules, modify the Passport object (e.g., `passport.use(...)`), and register a provider type.  I think we should create a dir `server/authentication` and have a few files in there:

* `server/authentication/index.js` - this will be a module that will require/initialize passport, figure out which provider to use based on the `env` settings, then dynamically load (i.e., require) the appropriate module, which will do the provider specific stuff.  If you look at how you use Facebook, for example:

http://passportjs.org/guide/facebook/

You need code that can do all that extra stuff for Facebook, which needs to live in a separate module.

* `server/authentication/passport-webmaker.js` - this is the default for webmaker.org, and it will work sort of like the Passport Local provider, but inspect the cookie and make sure we have the username attached.

I'm happy to sketch this out in more detail over video if you want.