Make config paths have some alternate locations

[mpdehaan]

Currently the app really wants to use "/etc/source_optics" as is specified in settings.

While the user CAN change this, settings.py is in source control, and "/etc/source_optics/conf.d" is NOT, so changing this means an update to the source code would put those changes back or risk getting included in a PR.

What I would suggest is that settings.py be coded to try to use "~/.source_optics" if it exists, and if NOT, then use the "/etc/" location.

We can also make the generate_secret management command work this way as well, as well as secret loading. In fact, we have to do both to finish this ticket out. It's a pretty easy code change.

@grayaii