Move settings into taccsite_cms/settings

[wesleyboar]

Summary

Move taccsite_cms/settings.py into taccsite_cms/settings/ for consistency with Core-Portal.

Proposed Changes

• Move the main settings implementation into the taccsite_cms/settings package (likely __init__.py, keeping DJANGO_SETTINGS_MODULE=taccsite_cms.settings).
• Remove fallback imports of taccsite_cms/settings/settings_*.py n' …secrets.py once clients have migrated.
• Update manage.py, wsgi.py, docs, and any deployment references if needed (depending on final layout).
• Document in docs/upgrade-project.md the required steps for clients.

Notes

• This should be a v5-only breaking change, after v4 has supported both old and new locations for at least one minor version.
• Coordinate with https://github.com/TACC/Core-Portal so that the layout matches its current pattern.