admin: ensure that article slugs are unique
admin: ensure that article slugs are unique

There was a logic error in this previously, causing it to only try
duplicate slug handling when there's not a duplicate. Fixing it this way
is probably better than letting people deal with duplicate slugs
themselves.

Fixes #151.

wuvt/wuvt-site,wuvt/wuvt-site,wuvt/wuvt-site,wuvt/wuvt-site