Add some admin theme callbacks and make them explicitly admin_xxx for consistency

with the callbacks in the xxx_block helpers.  So in the theme we have:

admin.html.php:
  $theme->admin_page_bottom()

then in the helpers:

core_block.php:
  function admin_page_bottom() { }

core/libraries/Theme_View.php

@@ -93,6 +93,7 @@ class Theme_View_Core extends View {
     case "album_bottom":
     case "album_top":
     case "credits";
+    case "footer":
     case "head":
     case "header_bottom":
     case "header_top":