WIP framework for adding data sources, about and data formatting pages, validation

This pull request is a work in progress.

* @evz is working on the framework for adding data sources. This framework is defined https://github.com/datamade/geomancer/blob/master/geo/utils/mancer.py and is now being inherited by https://github.com/datamade/geomancer/blob/master/geo/utils/census_reporter.py
* I've added content pages for about and data formats #20
* fleshing out some form validation #7, which will also help to implement moving back to previous steps in the wizard #21