Make the user_ip_counter index actually unique.
Make the user_ip_counter index actually unique.

I stupidly forgot to set that in the original migration that added this.
This fixes that, first cleaning up all the dupes that occurred in the
meantime.

Fixes #250.

ShieldBattery/ShieldBattery,ShieldBattery/ShieldBattery,ShieldBattery/ShieldBattery,ShieldBattery/ShieldBattery,ShieldBattery/ShieldBattery