topstories items are now cached in sessionStorage for fast-back

The rest of this happened by accident:

Split ListItem into DisplayListItem, TopStoryListItem and ListItemMixin
Consolidated renderItemMeta and renderItemTitle into ItemMixin
Extracted duplicated query param page calculation into PageNumberMixin
Added a mixins dir to contain the above
Extracted duplicated page calculations into pageCalc()
Removed items to TopStories
Extracted repeated constants into a constant module