Feature request: ability to specify additional CKAN User fields

It would be nice to add additional fields to the user profile, e.g. twitter, github, API keys to 3rd-party services, etc.

I imagine these additional fields can be used by CKAN extensions to implement additional functionality - e.g. tweeting when a dataset is updated using the user's twitter account, rendering a CartoDB viz using their CartoDB account, etc.

The issue names no files, tests, or entry points. Start by tracing how CKAN user profiles and schemas are represented, then inspect how extensions can access user data. Done means defining and documenting a general way to add fields such as social accounts or service credentials without limiting extension use cases.

Written by the indexing model from the issue text.