Embedding same note multiple times

[reefdog]

Sprung from: documentcloud/documentcloud#19

Been chatting with @knowtheory about ways to allow this. Basically, we should be able to internally store a note model once and then reference it from multiple note views. Currently, there's a 1:1 relationship and so the last view instantiated is the only one who gets the note. (dc.embed.notes is keyed by note ID and points to a noteView, so you can't have multiple noteViews modeled on the same note.)

Goals:

1. Make this work
2. Make this work in a way that doesn't break existing integrations

Classifying this as a bug because there are many reasonably scenarios where a user would expect the same note to appear on the same page (e.g., index page for blog posts which each embed the same note).