Add a Packages application and PackagePublisher

Summary:
Ref T8116. Partially scavenged from D14152. This roughs in a new Packages application for Arcanist extensions and third-party applications, and adds a "Publisher" object.

A "Publisher" represents an individual or entity who is publishing a package, like "Phacility". It's explicitly not necessarily the original author -- just the primary entity vouching for the safety of the code.

A publisher just has a name and a unique key for now. For example, Phacility might have "Phacility" and "phacility", respectively.

Unique keys are immutable, e.g., the package "phacility/arcanist" will always be exactly the same package by exactly the same publisher.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8116

Differential Revision: https://secure.phabricator.com/D16314