Made tag data save in a non-conflicting way.

Before 'tag/<tag_id>' is where the tag info was stored and
'tag/<tag_id>/<data_id>' is where the link to a data_id was stored. This
made it impossible to download for backup, syncing, or whatever as the
file name 'tag/<tag_id>' and the directory name would conflict.

So I changed it to use 'tag/<tag_id>/tag' for the tag info.