Fixed problem with env in settings.py and updated README about ALLOWED_HOSTS

[fklemme]

A couple of improvements from user feedback on discord.
Apparently, environment variables coming from env are string by default and not automatically cast to higher-level types such as lists. To ensure correct behavior, I rewrote the env lines in settings.py to only depend on strings.
I also added a small note on DJANGO_ALLOWED_HOSTS to the README as it caused some trouble in a setup.

[ndepaola]

i thought django-environ handled type casting of environment variables? https://django-environ.readthedocs.io/en/latest/

[fklemme]

I saw examples where the user configures explicit casting. But I think that will work only to some extend. Ofc, python will be able to handle something like int("2"). But I would not trust what happens to list("['a', 'b']"). But I'm also not a python expert, so I'm happy to be proven wrong.

[fklemme]

Okay, I see there is some "smart casting'. Lists need to be handled differently. Let me have another try on this one...

[fklemme]

Hmm, so after having a deeper look at smart casting and default values, I'm not even sure anymore if this is something we want to deal with. There are a lot of side effects and pit holes discussed on the django-environ github page. Apparently, these problems became even so dominant that smart scaling will be disabled in upcoming releases by default [1].

I agree that the code in this PR is not looking as smooth as the previous solution, but seeing the possible misinterpretations, I would consider it solving it as proposed for now.

Edit: Maybe one last note, because it might not have been clear in the description: The current implementation of ALLOWED_HOSTS is not working / cannot be changed through the .env file. So this implementation fixes that problem.