Added the necessary classes to send email summaries. First, added a
script to run on a daily basis. Second, added a queue handler for
sending email summaries for users, and another to queue summaries for
all users on the site. Fixed up the email_summary_status table to
store the last-sent notice id, rather than a datetime (since we don't
support 'since' parameters anymore). Finally, made the plugin class
load the right modules when needed.
