VID-1907 Logic to display the new galleries

This is what I was getting at with the comment for this line.  A gallery tag with no type, e.g.:
```php
<gallery> 
  foo.jpg
</gallery>
Is the default gallery type and is assigned the class type self::WIKIA_PHOTO_GALLERY.  Adding a new type and assigning type would make these mutually exclusive when we have default galleries that are also nagivational galleries.

This is confusing enough that I'm going to suggest changing how we mark galleries.