message_fetch: Use first unread anchor for home view.

This fixes one of our oldest important user experience issues, namely
that if you never visit the home view, the Zulip webapp would often
load "deep in the past" because the pointer had not advanced.

Fixes #1529.

frontend_tests/node_tests/message_fetch.js

@@ -138,7 +138,7 @@ function message_range(start, end) {
 const initialize_data = {
     initial_fetch: {
         req: {
-            anchor: 444,
+            anchor: 'first_unread',
             num_before: 200,
             num_after: 200,
             client_gravatar: true,

static/js/message_fetch.js

@@ -402,7 +402,9 @@ exports.initialize = function () {
         // near: narrow query, we want to select a specific message.
         anchor = page_params.initial_pointer;
     } else {
-        anchor = page_params.pointer;
+        // Otherwise, we should just use the first unread message in
+        // the user's unmuted history as our anchor.
+        anchor = "first_unread";
     }
     exports.load_messages({
         anchor: anchor,