CoinbaseController: add support for discounted prices.
Gold: support buying and redeeming gift codes
IpnController: cleanly exit if payment_blob is locked.

/u/reddit's inbox was full causing IpnController.finish to fail
on send_system_message, but after dishing out the gold. Temporarily
ignore "locked" payment_blobs.
ipn: protect against exceptions raised by send_system_message.

/u/reddit's inbox can get full.
_min_daily_pageviews_by_sr: get_time_points takes datetime arguments.
Gold purchase amount dropdowns: reduce choices
Delete SaveHide.
Delete unused Link _saved and _hidden methods.
Delete update_promos_q.

A cron job is sufficient because promote.make_daily_promotions()
is all that needs to be run.
paypal subscription: cleanly exit on duplicate transactions.