Use SECRET_KEY code from rpahendyr-django-essentials

[raphendyr]

Description

Copied Django easy-of-life changes from A+/MOOC-Grader to Radar. I haven't tested these, but I think they should work (thus the PR).

This also includes environment variables to make container based development easier.

Testing

Check that the Django stil lstarts ok and the local_settings.py is found correctly.

Is it Done?

Clean up your git commit history before submitting the pull request!

• [ ] I (developer) have created unit tests and the tests pass
• [ ] I (developer) have created functional tests (Selenium tests) if applicable
• [ ] I (developer) have tested the changes manually
• [ ] Reviewer has finished the code review
• [ ] After the review, the developer has made changes accordingly
• [ ] Customer/Teacher has accepted the implementation of the feature

[raphendyr]

FYI: I started to make container usage a bit easier with this commit, but I didn't have time to finalize it.

The goal was to make settings look like

update_settings_with_file(__name__, 'local_settings', from_environ='RADAR_LOCAL_SETTINGS')
update_secret_from_file(__name__, 'secret_key', from_environ='RADAR_SECRET_KEY_FILE')

It's still missing the quiet part.

[Gehock]

I tested the changes, rebased on top of the current head and slightly updated the documentation here: https://github.com/gehock/radar/tree/auto-secret-key

I don't have the permission to force-push into this current PR, does somebody with permissions want to do that themselves or should I open a new PR?

[markkuriekkinen]

@Gehock I can merge the code now. No need for a new PR. Thanks!