Simplify getting imageable/documentable in forms

The imageable/documentable object is always the object the form builder
is based on; since we're already passing the form builder, we don't have
to pass the object as well.

The only exception are the poll answers. In this case, we're passing a
new answer as the object. That's OK; the same hack that we're using to
send the data to the answer URL without displaying existing attachments
causes the form to keep working the same way.

app/views/documents/_nested_documents.html.erb

@@ -1,3 +1,5 @@
+<% documentable = f.object %>
+
 <div class="documents-list">
   <%= f.label :documents, t("documents.form.title") %>
   <p class="help-text"><%= documentables_note(documentable) %></p>