dbt coding conventions 2.0

Update of our dbt coding conventions on 2021-08-06.

• Specify dbt project structure (@NiallRees).
• Add marts directory (@NiallRees).
• Rename "staging" models "intermediate" models with an int_ prefix (@NiallRees).
• Add source_ prefix to source models (@NiallRees).
• Separate models' output columns into logical groups (@sean-rose).
• Don't use fact_, dim_, or agg_ prefixes for model names (@sean-rose).
• Cast timestamps to the client's timezone (@jaypeedevlin).
• Require alphabetical order in YAML files (@jaypeedevlin).
• Document source models and sources (@jaypeedevlin).