Move social auth to INSTALLED_APPS

Currently, `social` is included in `INSTALLED_APPS` only when third-party auth is enabled.  When installed, this app creates several database tables.

To make the deployment process simpler, I've moved the `social` app into `INSTALLED_APPS`, so that the database tables can be created even if the feature flag is disabled.

The alternative is to temporarily set `FEATURES['ENABLE_THIRD_PARTY_AUTH']` to True on the server where you run `syncdb --migrate`.  We would need to do that before enabling the feature flag on the application servers.

@feanil @flowerhack please review.