unread: Stop treating bottom of render windows as the global bottom.

The previous logic for both scrolling down and using pagedown would
incorrectly mark an entire conversation as read when reaching the
bottom of a render window, even if there were more messages loaded or
to fetch from the server.

Fix this error in the calculation by asking the correct data
structures if we're actually at the bottom.

To avoid the navigate.js keyboard shortcut code paths circumventing
this new logic, or needing to duplicate it, they now call
process_visible, rather than its helper.

web/src/message_list_view.js

@@ -1439,6 +1439,18 @@ export class MessageListView {
         this.message_containers.clear();
     }
 
+    is_end_rendered() {
+        // Used as a helper in checks for whether a given scroll
+        // position is actually the very end of this view. It could
+        // fail to be for two reasons: Either some newer messages are
+        // not rendered due to a render window, or we haven't finished
+        // fetching the newest messages for this view from the server.
+        return (
+            this._render_win_end === this.list.num_items() &&
+            this.list.data.fetch_status.has_found_newest()
+        );
+    }
+
     get_row(id) {
         const $row = this._rows.get(id);