External folder handling for files

Models, Vaes, nodes setups, inputs, outputs, etc. are currently stored in the ComfyUI folder in predetermined locations. Any files not part of the install itself should be able to be moved another location, separate from the install folder. This has 2 key advantages:

- It allows you to put these files on a separate drive/device if needed for space (due to the large sizes that can be involved) or technology (SSD vs HDD, NAS use, separating data and programs across different drives) reasons without resorting to symlinks.
- It separates these files from the install so that, in the event something goes wrong that warrants a full delete and reinstall, the files that are not part of the install itself are not inside the folder structure of the install, leading to them also being deleted unless the user remembers to manually move each and every relevant file/folder, which is likely to result in accidental deletion of at least some files.