store logs per-edition instead of per-package
- [x] define a “most recent build result” metadata structure
- [x] define a collection for the aformentioned metadata
- [x] add logic for computing a date-sharded S3 URL for each log
- [x] add logic for loading the build metadata and displaying it in the web UI
- [x] define DB index for querying logs by date
- [x] deprecate and decommission the old build metadata system
- [x] delete previous-generation logs from S3
- [ ] implement plugin for deleting build logs after some retention period (30d?)