read/unread indicators

[jeremybmerrill]

it'd be great (and easy) to add a read/unread flag to documents in search results.

could keep it in localstorage.

one of the key learnings here is that "reporters just wanna read" and that part of the goal of searching is to make the haystack a little smaller. since one document will likely come up in multiple searches, it'll be useful to flag certain documents as already having been seen.