Snippets with comments cause a 500 when they show up in search results

This occurs when you search for a snippet that has a comment. Here's how to reproduce:

1. Create a snippet.
2. In the comment field, enter "Supercalifragilisticexpialidocious".
3. Search for "Supercalifragilisticexpialidocious".
4. Click on "Comments".

-Stan

/cc: @rspeicher

https://sentry.gitlap.com/gitlab/gitlabcom/issues/3643/

NoMethodError: undefined method `iid' for #<ProjectSnippet:0x007faa06841cc0>
  app/views/search/results/_note.html.haml:18:in `_app_views_search_results__note_html_haml__3011065897183706563_70184116382260'
    %span #{note.noteable_type.titleize} ##{note.noteable.iid}
  app/views/search/_results.html.haml:20:in `_app_views_search__results_html_haml___3568432111070933049_70184117488600'
    = render partial: "search/results/#{@scope.singularize}", collection: @objects
  app/views/search/show.html.haml:7:in `_app_views_search_show_html_haml__2673936590556281126_48154940'
    = render 'search/results'
  lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/metrics/transaction.rb:45:in `run'
    yield
...
(163 additional frame(s) were not displayed)

ActionView::Template::Error: undefined method `iid' for #<ProjectSnippet:0x007faa06841cc0>
  app/views/search/results/_note.html.haml:18:in `_app_views_search_results__note_html_haml__3011065897183706563_70184116382260'
    %span #{note.noteable_type.titleize} ##{note.noteable.iid}
  app/views/search/_results.html.haml:20:in `_app_views_search__results_html_haml___3568432111070933049_70184117488600'
    = render partial: "search/results/#{@scope.singularize}", collection: @objects
  app/views/search/show.html.haml:7:in `_app_views_search_show_html_haml__2673936590556281126_48154940'
    = render 'search/results'
  lib/gitlab/metrics/rack_middleware.rb:17:in `block in call'
    retval = trans.run { @app.call(env) }
  lib/gitlab/metrics/transaction.rb:45:in `run'
    yield
...
(163 additional frame(s) were not displayed)