Peace and harmony, in all the URLs

* Make bulk add and import cope with new_urls up to 2048 characters in length.
mySQL used to silently truncate long new_urls, Postgres is more honest.
* Be sort-of-consistent and allow 2048 characters in `mappings_path` (not quite consistent because there's no scheme or host, but at least we only have to remember one number)
* Drop `mappings_staging`. It's no longer used.