ERD (Entity Relationship Diagram) 1.0.0

Use this PR to define the fields and entities for the project.  

The old PR is linked here for reference: https://github.com/sfphp/sfphp.org/pull/15

When this PR is done all commits should be rebased to a single ERD commit and that will be the starting point of the application.  Future changes to the ERD will be tracked through other PRs.