Add a link to edit a text shout

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The error message undefined method `model_name' for TextShout::ActiveRecord_Relation:Class
keeps appearing no matter how I approach the problem.

1) Methods used: edit_polymorphic_path(shout)
2) Exclude resources :content, nested under both text_shouts and photo_shouts
3) ActivModel::Naming

And some others.

@gabebw
`find_by` was a great call. I got it to work by changing the code in `TextShout#edit` to:

`@text_shout = Shout.find_by(content_type: "TextShout", content_id: params[:id])`

And the view to:
```ruby
<%= form_for "TextShout" do |form| %>
  <%= form.text_field :body %>
  <%= form.submit "SHOUT!" %>
<% end %>
```

That seems a little sloppy though. Is there a better way?