Allow admins to transfer cookbook ownership

:fork_and_knife: Implement the ability for Supermarket admins to transfer the ownership of a cookbook to any other user.
To me (and it could be just me) it's confusing to have a parameter called `include_collaborators` which when blank results in a method named `eligible_collaborators` being called. Looking at it from a slightly wider lens, I might propose eliminating the parameter and the new method, and pushing any conditionals into `eligible_collaborators`:

```ruby
def eligible_collaborators
  if policy(@cookbook).transfer_ownership?
    User.where('users.id != ?', @cookbook.user_id)
  else
    ineligible_users = [@cookbook.collaborators, @cookbook.owner].flatten
    User.where('users.id NOT IN (?)', ineligible_users)
  end
end
```

I think this captures the intent of the conditional a bit more directly.