Investigate Django settings best practices / patterns / libraries etc

Would be worth looking around for projects, or best practices we can learn from

One of the current pains is that if you have eg:

```
ANNOTATION_BASE_DIR = "/data/annotation"
ANNOTATION_VEP_BASE_DIR = os.path.join(ANNOTATION_BASE_DIR, "VEP")
ANNOTATION_VEP_VERSION_DIR = os.path.join(ANNOTATION_VEP_BASE_DIR, "vep_code", ANNOTATION_VEP_VERSION)
```

and you want to change ```ANNOTATION_BASE_DIR``` you have to re-declare everything that depends on it

There might be some way to get around this like evaluating at run time, or being immutable and setting 1st - check for what others do