Subscribe adopters

This branch adds a rake task to subscribe recent drain adopters. The task first finds all users who have adopted at least one drain and for whom the subscribed_at column is null. It then attempts to subscribe them via the specified host. If successful (better error handling is needed here), the subscribed_at column is set to the current time.

* added migration for users.subscribed_at column to track whether a user has already been subscribed
* gratuitous fixes of honolulu.gov to oaklandnet.com or adoptadrainoakland.com

Format of rake task is:

rake subscribe_recent_adopters[HOSTNAME,API_KEY]

Contact me for the appropriate values for HOSTNAME and API_KEY for the staging and production servers.