Use the Last-Modified header's date for the index refresh timer.

Instead of showing how long has it been since we refreshed the index,
we show how long has it been since the index has changed on 4chan's side.
That'll be a more meaningful timer when 4chan freezes for example.

src/General/Index.coffee

@@ -249,7 +249,7 @@ Index =
       setTimeout notice.close, $.SECOND
 
     timeEl = $ '#index-last-refresh', Index.navLinks
-    timeEl.dataset.utc = e.timeStamp <% if (type === 'userscript') { %>/ 1000<% } %>
+    timeEl.dataset.utc = Date.parse req.getResponseHeader 'Last-Modified'
     RelativeDates.update timeEl
     Index.scrollToIndex()
   parse: (pages, pageNum) ->