Fix N+1 query when checking authorisation policies for issues and comments

[redrabbit]

In commit 9e30f7e, we introduced more complex rules for both GitGud.Comment and GitGud.Issue when passing the :admin action. The new rule retrieves the repository maintainers and returns true if the user has at least write permission.

What we basically want to achieve here is redirecting the policy to a different resource:

 # Maintainers with at least write permission can admin the resource.
 def can?(%{repo: %Repo{} = repo}, user, :admin), do: authorized?(user, repo, :write)
 def can?(%{repo_id: repo_id}, %User{} = user, :admin) do
   if maintainer = Repo.maintainer(repo_id, user),
    do: maintainer.permission in ["write", "admin"],
   else: false
end

• The first match is executed if the resource has already preloaded :repo and forwards to the repo authorisation policy.
• The second is a required workaround if :repo has not been preloaded...

Now let's see why this implementation is problematic... Say we have a comment thread (issue, commit review) and we want to check for :admin permission for each comment. Calling authorized?/3 for each comment would result in a N+1 query.