Find the config folder

To access the config folder simply open the Settings [Ctrl] + [,] and click on the last item under the menu: "Open Config Folder". There you can Right Click on the root of the project and "Show in file manager".

Save / Reinstall the packages

I created the "package-file.list" file following the instructions of this stackoverflow question.

Open a terminal and cd into the config folder.
Backup: apm list --installed --bare > package-file.list
Restore: apm install --packages-file package-file.list