move Postgres settings into separate settings class

[hrodmn]

Related Issue(s):

• resolves cannot load app module without setting Postgres environment variables #208

Description:

Move the Postgres settings out of the main Settings class into a separate PostgresSettings class, then defer loading the Postgres settings until connecting to the database

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.