Add repo cleanup tasks

* `repo:cleanup_orphans` to remove repos with no memberships.
* `repo:cleanup_duplicate_github_ids` to remove duplicate repos
  with the same `github_id`.
How about using [`update_attribute`](http://api.rubyonrails.org/classes/ActiveRecord/Persistence.html#method-i-update_attribute) instead (it skips validations)?
```ruby
create(:repo).tap do |repo|
  repo.update_attribute(github_id: r1.github_id)
end
```