Simplify Podman structure with quadlets

[JetstreamLarry]

Currently, everything is a pod defined in pod.yaml files, which unnecessarily complicates things since:

1. .yaml files are not easy to read and edit (especially in the cli)
2. Almost nothing needs to be in a pod (exceptions are mail, wordpress, etc)

Once we move to quadlets (see #30), this can be simplified by using .container files, which are:

1. Easier to read and edit
2. Easier to put into pods when necessary using .pod files

This would also likely help with #8