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
Also: does `TextShout` have `content_type` and `content_id` columns? It looks like those are actually on `Shout`.