[Tilesathome] Effects of single-tileset upload change

[OJW]

When the uploads start to store just one piece of metadata per tileset, there 
will be a few visible changes:

* metadata-viewing API won't return accurate results until it gets new 
functionality to tell it about tileset credits
* downloadable list of tiles will probably be misleading, initially

Internally, there are some changes of interest to server developers:
* Metadata won't be available for all tiles (obviously)
* Filesize not stored in the database for tiles
* More complex code required to find meta-info for any given tile (have to 
search the z-12 as well as the tile itself)