Add Stats endpoints, settings and management commands

[cduhn17]

🗣 Description

Merge the methods to get stats infor for the frontend into the main backend python branch. Also includes includes updates to settings to centralize env variables to be used in API endpoints, tasks that will be run to populate AWS elasticache and supporting management commands that if in the event we need to manually push new data to elasticache

💭 Motivation and context

This change is required to add functionality to the stats endpoints.

🧪 Testing

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All future TODOs are captured in issues, which are referenced in code comments.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.

✅ Pre-merge checklist

• [x] Revert dependencies to default branches.
• [x] Finalize version.

✅ Post-merge checklist

• [x] Create a release.