Rework Egyptian Revivalist Acquisition event

1. Add a mechanism to generateRelatedSlave to produce an opposite-sex relative instead of a same-sex one (rather more thorough than what the ERA event was doing...fixes sex-linked genetic quirks and so on).
2. Make the ERA event use generateRelatedSlave.
3. Hook it up to Bulk Slave Intro.
4. Slightly alters the Bulk Slave Intro boilerplate used in both HL and the ERA event to avoid polluting $newSlaves if the purchase is declined.
5. Delete the ERA workaround passage.