Update sample project resources for fides evaluate usage in fides deploy

[NevilleS]

Code Changes

• Fix reference to non-existent dataset demo_users_dataset
• Update Postgres & Mongo systems to be specified as Databases
• Update Postgres & Mongo system declarations to be more realistic uses 😄

Steps to Confirm

• Run nox -s "build(sample)" to build sample project files
• Run pip install ~/git/fides && fides deploy up --no-pull in a venv to install locally
• Try fides pull -a .fides/sample_resources.yml to pull all resources
• Try fides evaluate --dry to see if policy evaluation passes by default
• Modify the cookie_house_mongo declaration to be used for advertising.third_party and see if policy evaluation fails

Pre-Merge Checklist

• All CI Pipelines Succeeded
• Documentation:
  • documentation complete, PR opened in fidesdocs
  • documentation issue created in fidesdocs
• Issue Requirements are Met
• Relevant Follow-Up Issues Created
• Update CHANGELOG.md

Description Of Changes

This modifies the Cookie House YAML declarations to be more accurate and consistent, removes the reference to the missing demo_users_dataset, and makes it easier to quickly experiment with Fides policies!

Goal here is just to have a built-in demo of the CI policies baked into the Cookie House sample project too - we're close, but since the declarations aren't quite internally consistent it doesn't serve as a useful demo of those features yet.

[ThomasLaPiana]

Confirmed the failure, nice change!