Move helm metadatum to the package level

🔥 Current situation

The helm metadatum is currently linked to package files. The problem is that we allow duplicated uploads to the helm registry.

Those duplicated uploads will create new rows for the metadatum. The issue is that those rows will all be super similar.

🚒 Solution

It could be better to hold a metadatum object linked to the package and use that as a the single location for the metadatum.

When a background job processes a chart upload, it will simply update the metadatum at the package level.

This gives us a single metadatum row for each helm package (vs having n metadatum rows for each helm packages).

⚠️ We will need a migration for the existing data.