Throw in quick hack $wgRateLimitsExcludedIPs so we don't have to do a live hack

Throw in quick hack $wgRateLimitsExcludedIPs so we don't have to do a live hack
every time somebody runs an exciting event where they're going to register
a lot of accounts.
Should be killed and replaced with an on-wiki system for approvals or temporary lifting of limits as a sort of anti-block.