core:backend: enable overriding BASE_DIR setting

[juliamrch]

Purpose

Django backend currently collects static files and creates a /data directory to store them. DATA_DIR path is hardcoded as / and that can cause some systems to throw errors (Mac, PaaS, VMs and containers, and other systems with strict read-only permissions ). The error will look like this:

    os.makedirs(directory, exist_ok=True)
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen os>", line 217, in makedirs
  File "<frozen os>", line 217, in makedirs
  File "<frozen os>", line 227, in makedirs
OSError: [Errno 30] Read-only file system: '/data'

Changes in this PR allow developers to override the path in order to test the backend in any environment.

Proposal

I propose to avoid hardcoding the backend root as an absolute system path and replace it by the variable BASE_DIR, which is a standard Django convention. DATA_DIR will always point to Django project's root directory by default to create /data, unless specified otherwise.

Note: similar to #893 but with a different method.

[soyouzpanda]

I think the method used in #893 is better because you rely on the fact that BASE_DIR/data is writeable, which will not be the case in a system using NixOS (BASE_DIR will be referencing /nix/store)

[juliamrch]

I think the method used in #893 is better because you rely on the fact that BASE_DIR/data is writeable, which will not be the case in a system using NixOS (BASE_DIR will be referencing /nix/store)

I agree with your statement @soyouzpanda, but since it was hardcoded I sticked with the original design because I honestly wasn't completely sure about the necessity of sticking with the designated path.

This PR just stops DATA_DIR from pointing at some systems root and keeps it in what I concluded was the designated path on a UNIX OS. #893 provides more flexibility, and fixes both Nix and the current issue this PR adresses, so I'll let the reviewers choose which one suits this project better.