Add string start_date and end_date properties to PromoCampaign.

The format makes the strings sortable so they can be used for
querying.
Add daily_bid property to PromoCampaign.

This is needed to replace PromotionWeights.
Tests and backfill for PromoCampaign string date properties.
Stop reading from PromotionWeights.
Delete PromotionWeights.
Promote_Graph.get_current_promos return campaign and don't give dates.

No one was using the dates anyways.