Allow users to set OPENAI_API_KEY in the app config

[kasnerz]

Currently, we expect OPENAI_API_KEY to be set as environmental variable, which can be non-intuitive.

We can allow the users to set this key (and also other API keys) in the app configuration file, so that it can be set through the web interface or the YAML config file.

Are there any safety issues that we should think of? @oplatek

[oplatek]

We should not log the variable or the whole environment (even when raising some unexpected exception) otherwise we are fine.

And we should create the file with 600 permisions

[oplatek]

I think it make sense to load the api_key from the Metric config as well

[kasnerz]

But then you'll have to set the key every time you start a new campaign, which seems a bit inconvenient 🤔