Improve copy around managing attachments

This improves the copy around managing attachments on the editions form and makes the govspeak help contextual so it shows an appropriate message based on what is being edited.

With regards to the govspeak markdown help, we have to deal with the fact that not all models use the new attachments tab for managing attachments. We therefore show different copy based on the following conditions:

  1. Editions that do not support inline attachments (solution: don't show any attachment-related markdown help; it's not useful)
  2. Editions that do support inline attachments (solution: show the markdown help and mention the attachments tab)
  3. Other models that support attachments using the old UI (solution: show the markdown help and don't mention the attachments tab)

Ideally, the message would be tailored based on whether you are creating a new edition or editing an existing one, but this is good enough for now as it is just a stop-gap until all models are using a consistent interface for managing attachments.

https://www.pivotaltracker.com/story/show/54096855