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).

db/migrate/20191223032633_add_is_deleted_to_user_feedback.rb

@@ -0,0 +1,5 @@
+class AddIsDeletedToUserFeedback < ActiveRecord::Migration[6.0]
+  def change
+    add_column :user_feedback, :is_deleted, :boolean, default: false, null: false
+  end
+end