Consider restoring the ability to log in to PlantShare using Facebook

[jnga]

PlantShare originally had the ability to log in (or sign up?) using Facebook, added with #205. Recently it was discovered that the feature seemed to be no longer functioning in Production. In addition, the project being used appeared to be no longer actively maintained, and there were problems getting recent versions of pip to install the package from GitHub.

So, as part of fixing Dev deployments with #628, the Facebook login feature was removed. Decide whether this capability would still be useful, and if so, look for an actively maintained project that works with >= Django 1.8 for restoring it to the site.