Move back-end download functions to /api/sedfittingresult

[Abh1ramManugur1]

Resolves #411

• Added new download functions to /api/sedfittingresult, replacing view functions: download_chains, download_modelfit, download_percentiles
• New download URL: /api/sedfittingresult/{id}/download/{download-type}/
• Fixed download URLs on /api/sedfittingresult using new download functions, rerouted transient download on front-end to these functions. Functionality from front-end is identical

[manning-ncsa]

When I query example endpoint http://localhost:4000/api/sedfittingresult/?transient=2025ass&aperture_type=local, the returned data looks like

[
    {
        "id": 7,
        "chains_file": "http://app/api/sedfittingresult/7/download/chains/",
        "model_file": "http://app/api/sedfittingresult/7/download/model/",
        "percentiles_file": "http://app/api/sedfittingresult/7/download/percentiles/",
        "log_mass_16": 8.20374421964611,
...

It looks like Django is using the Docker Compose service name app as the domain when constructing the URL. We need to fix the URL building functions to generate a valid download link. While we're at it, these three functions should be refactored into a single function with the file type as an input argument.