bootstrap_flash and add_breadcrumb fixes

1. Fix exception being caused by use of `public_send` in add_breadcrumb macro.
2. Don't attempt to display a flash message if the message can't self-convert to a string.
    - Example: `flash[:notice] = true`. There's no point displaying this flash message in the page. Even if we did explicitly convert it to `"true"` that wouldn't be a useful message.