Create UI element for indicating source of annotation

[RichardLitt]

Source is just which paper it was first started on. When you start a conversation I think there should be the option for its source to be the currently viewed paper or just for it to have no particular source -- it could mention many papers but have none be the source. Imagine you're at buypizza.com and you start a beagle conversation about something, perhaps linking to various other conversations or something, then it doesn't have a source as such, since you were on a pizza website when you started it. I think you can make a note without it being associated formally with the present document in any way. ~ @adammarblestone