For security reasons, we need the ability to control the plugins that are
available through the update center. Additionally, we've got our own package
management infrastructure we'd like to use to host plugins.

This patch adds hooks to UpdateCenter to:

• Allow the server that update-center.json is retrieved from to be configured
• Allow customization of various aspects of downloading a plugin/upgrade:
• connection check
• download
• install
• upgrade
• Allows plugins to be hosted on a different server than update-center.json
• Passes the authentication object of the user that initiated the job to the
  background threads for auditing purposes.