adds migrations for collection and item metadata tables. posters (items + collections), wallpapers,
and themes now have a hash calculated based on their variables. adds check before generating poster,
comparing metadata slug to stored slug from previous sync, if both hash and slug are equal, then
generation/upload is skipped. will significantly reduce bloat from repeated uploads, and reduces
poster overlays re-run time significantly (e.g. 45 mins to 18 mins)
re #144
