`HasWiki#wiki` is broken

Summary

HasWiki#wiki builds wiki with object.owner, but Project#owner is not guaranteed to return a user (for any project in a group it returns the group) and for groups it mostly returns nonsense, since groups have a distinct concept of ownership.

Steps to reproduce

In a console in the GDK dev environment:

p = Project.first
w = p.wiki
wp = w.find_page('home')
wp.update(title: 'zuhause')

Observe the exception.

Example Project

This happens for all projects that belong to a group.

Possible fixes

Replace self.owner with a different concept of ownership, guaranteed to return a User.