Make OverrideThrottle handle modifications of existing throttles
If a user enters a target for which there is an existing throttle,
show the user an error and let the user resubmit the form if the
existing rule needs to be modified.
Database table changes
- Add thr_target which is indexed
- If no throttle type is given, don't insert it.
- Removed dodgy range check; looks like IP::isIPAddress() does that check