Transfer ownership to another user

This allows a site administrator to transfer ownership of a plugin (and all releases) to another user.

This is the first step toward being able to have willing users assume ownership of abandoned plugins, or (as in what I need to do) transfer plugins from a personal account to an organization-owned account.

Currently restricted to site admins, but can easily be extended for plugin owners to transfer the plugin away from themselves - I didn't go this route yet as there may need to be discussion about a workflow for that.

eg.
Can they transfer to any arbitrary user?
Should that user have the option to accept/reject the transfer?
etc.