user feedbacks: replace permanent deletions with soft deletions.

* Add is_deleted flag.
* Allow mods to delete and undelete user feedbacks.
* Don't hide old name change feedbacks (these will be deleted instead).

app/views/user_feedbacks/edit.html.erb

@@ -10,6 +10,9 @@
     <%= simple_form_for(@user_feedback) do |f| %>
       <%= f.input :category, :collection => ["positive", "neutral", "negative"], :include_blank => false %>
       <%= dtext_field "user_feedback", "body" %>
+      <% if @user_feedback.deletable_by?(CurrentUser.user) %>
+        <%= f.input :is_deleted, label: "Deleted" %>
+      <% end %>
       <%= f.button :submit, "Submit" %>
       <%= dtext_preview_button "user_feedback", "body" %>
     <% end %>