srnames_from_site: PromoCampaigns are now targeted to Frontpage.name.

They were previously targeted to empty string to indicate the Frontpage.
Avoid using '' for Frontpage subreddit name.

Some occurrences haven't been detangled yet and are now noted.
get_campaigns_by_date: fix ignore.
inventory: Delete unused get_predicted_by_date.
inventory.get_campaigns_by_date: don't separate campaigns by sr.

Previously the campaign by date dict was contained within a by
subreddit name dict.
Stop pretending that get_predicted_pageviews is date dependent.
get_campaigns_by_date: remove deleted campaigns.
Use an algorithm to determine free inventory.