Avoid outputting href="None" links in the admin for non-routable pages

There are a number of situations where a page is considered live but is not accessible at any URL: namely, when the page is outside of any site root, or (as of #4276) the wagtail_serve URL route has been removed. Various templates in the admin do not check this when linking to live pages, resulting in links that look like <a href="None"> - ones that I found in a very brief search were:

• wagtail/admin/templates/wagtailadmin/home/recent_edits.html
• wagtail/admin/templates/wagtailadmin/shared/page_status_tag.html
• the wagtail.contrib.sitemaps app

We should do a more thorough search for these, and fix them where found: ‘view live’ action buttons (as in recent_edits.html) should not be displayed if there is no URL, and status indicators (as in page_status_tag.html) should be displayed but not linked.

[lb-]

Go for it @SanskarGosavi2003 - I don’t think the previous person submitted a PR.

[gasman]

@karranb Yes, please do!