Note persistent id linkages

From the Pivotal comment:

We've implemented this as follows:

  * note_index_item.reference can contain a pointer to the persistent_id of another note within the same record.  We used the persistent_id since there's no ref_id in notes that we could see.

  * We added persistent_id to the forms for all note types, since they weren't previously being displayed.

  * We automatically generate a value for persistent_id if one isn't given when saving a note.

  * In the public interface, we show a note_item's reference as a hyperlink if we're able to match it to the persistent_id of a note. That lets you click on the reference and jump straight to the note it refers to.