Feature/authorize apps

@hannan All the changes we discussed look good to me. I'm having a further thought about the feature description though.

I think you can tweak the feature description to be more along the lines of the following:

```
In order to use Login as an OAuth2 provider for NYU Libraries service applications
As an admin
I want to be able to add authorized client applications through a web interface in Login.
```

And then maybe also since the name of the file is `authorize_apps` and that really is the feature you're talking about the feature name could be `Feature: Authorizing (OAuth2) client applications`, not sure if the OAuth2 is necessary.

@sdalton, @hannan What do you guys think of this?