Added file fields for a release's tarball and checksum

[bmispelon]

This is in preparation for https://code.djangoproject.com/ticket/35980 which will likely change the naming scheme of build files (tarballs and checksums).

To prepare the migration operation I looked in the production server, dumped all versions, tarballs, and checksums and semi-manually wrote the mapping between the two.

TODO:

• Fix tests (Release objects created in tests need a tarball attached to them
• Make checksum a mandatory field in the admin form (it's only optional for legacy releases)
• Figure out if we also want a field for the wheel file. They're not visible on the site but are uploaded as part of the documented release process (and included in the checksum file).

[nessita]

Overall looks good! Added a comment, but I think it makes sense.

[nessita]

Could we programmatically generate this list instead of defining line by line? And if there are a couple of exception to the rule, let's only define that as an explicit dict.

I think we could use F on the update to use each row major, minor, and version? Something like (untested):

Release.objects.update(
    tarball=Concat(
        Value("releases/"), F("major"), Value("."), F("minor"), Value("/Django-"), F("version"), Value(".tar.gz")
    )
)

[nessita]

On a second look, and after some local testing, I wonder why we don't spare the new fields and just make them a property? That seems simpler and that it scales better...