Fix Sitemap Add External Link Display Order

In the sitemap when adding an external link the display order is not
updated so unexpected results can occur such as ending up second from
the top and other order issues.

Runs `Page::movePageDisplayOrderToBottom();` on newly created page to
move new external link to bottom of order as is the case with newly
created pages and update all sibling's order
Merge pull request #918 from mkly/bugfix_external_link_add_display_order_bottom

Fix Sitemap Add External Link Display Order