Most of the model logic pertaining to unread counts had been in
zephyr.js, along with a couple global variables. Now the code
is encapsulated in unread.js. It was a pretty straightforward
extraction with some minor method name changes. Also, a small
bit of the logic had also been in stream_list.js.
Conflicts:
tools/jslint/check-all.js
(imported from commit f0abdd48f26ab20c5beaef203479eb5a70dacfff)
