[WIP] Customizable theming fix background images issue

[superryeti]

Closes #1996

Background: On PR #1986, we started to allow specific deployments of Physionet customize the theme of platform. This involved making the css styles, and static images configurable by using the .env file.

This #1986, introduced few bad designs and issues as pointed out by #1989 (tracked css files are being edited), and #1995 (downloading of files during installation).
This PR uses the suggestion provided by @alistairewj on #1996

Context:
This PR allows us to modify/customize the static files without messing with tracked file. As suggested by @alistairewj , we added a new directory static-overrides(where the updated static files are saved during compilestatic command)

STATICFILES_DIRS = [
    os.path.join(BASE_DIR, 'static-overrides'),
    os.path.join(BASE_DIR, 'static'),
]

So when the collectstatic command is ran it will first look inside the static-overrides directory, and copy the files to the STATIC_ROOT. As mentioned in https://docs.djangoproject.com/en/dev/howto/static-files/#serving-static-files-during-development, the files in static-overrides will be used in case we have files with same name in static-overrides and static.

During deployment, It is expected that compilestatic command will be ran first(which will generate the css, image files and store them to the static-override folder), and then collectstatic command will be ran which will collect static files
from static and static-override directories

Note: Regarding the part of code which lets us customize background image, i also added the option to allow someone to provide a link to background image because i am not sure if we can provide a local image path where the deployment happens in a container.

WIP: Currently, when the static files are generated in static-overrides by compilestatic and copied to STATIC_ROOT by collectstatic command. The static files are still loaded directly from static instead of STATIC_ROOT directory.(Happens when i run the dev environment through docker).
I tried to play around with DEBUG, STATIC_ROOT, and STATIC_URL in local dev settings.