LinkedIn Integration (WIP--Do not merge)

This pull request adds an optional feature which integrates the edX platform with LinkedIn for the purposes of allowing students who have earned certificates to publish their certificates on their LinkedIn profile.

Information on usage is included in:

https://github.com/jazkarta/edx-platform/blob/jazkarta-linkedin/lms/djangoapps/linkedin/README.rst

For review by @dianakhuang and @ormsbee.