date_row: Update color via update_table_stream_color.

Previously, the function update_table_stream_color did not update the
color of the date_row when it appeared within a topic.  This was
incorrect because it meant that a small piece of the border to the
left of the message box would be left to the previous color when the
stream color was updated.  This commit resolves the issue by adding an
update.

Fixes zulip#15197.

static/js/stream_color.js

@@ -16,6 +16,7 @@ function update_table_stream_color(table, stream_name, color) {
         if ($.trim($label.text()) === stream_name) {
             const messages = $label.closest(".recipient_row").children(".message_row");
             messages.children(".messagebox").css("box-shadow", "inset 2px 0px 0px 0px " + style + ", -1px 0px 0px 0px " + style);
+            messages.children(".date_row").css("box-shadow", "inset 2px 0px 0px 0px " + style + ", -1px 0px 0px 0px " + style);
             $label.css({background: style,
                         "border-left-color": style});
             $label.removeClass(exports.color_classes);