(GH-98) Add package review comments

This way when a package gets rejected, we've captured the reasoning for the
rejection.
(GH-98) Ensure approved date is null

If you resubmit a package at the same time we mark it approved, ensure that
approved date goes back to null
(GH-98) add package review status migrations to proj
(GH-98) Add package review fields to PackageViewModel

This adds the fields for package review and approval to show up on the package
page.
(GH-98) Show status on package page

This allows the following to be shown:
 - When a package has never been reviewed but is listed (all existing packages
prior to moderation), a big red warning shows up.
- When a package is approved, a message of moderator and approved date.
- Enhance the package under review message for the maintainer. If there are
review comments, have them show up.
- When a package is rejected, this shows to the maintainer with the specific
reviewer comments on why it was rejected.
- For moderators, display instructions on how to moderate a package.