Editable Metadata (web code)

Fixes #807 allow package metadata to be editable

Deployment process:
1) Run migration (copies data into new PackageMetadatas table) (QA: test the migration does not break old NuGet Gallery
2) Deploy new code
3) Rerun again the bit of SQL from migration which ensures the PackageMetadatas table is up to date with any packages uploaded before the new code was fully deployed.
4) And finally can run SQL to delete the PackageAuthors table, since it is no longer being used.