Create ResultSet method to list reports that should be mailed

[preaction]

Write a method on CPAN::Testers::Schema::ResultSet::Stats that will return a hashref of arrayrefs of stats rows to send in an e-mail to the given author, after following the preferences of the author (#32).

• since - A date/time to start collecting reports from. Will be the last run of the mailer.
• report_type - Which report type we're running. See Copy mail report preferences tables to main schema #32 for a list of valid report types.

This method should fetch the author and distribution preferences, loop through all the requested report summary rows, and return a hashref of author => reports to send to the author for this report type.

This is the most complex part of the current reports mailer code: Selecting which reports to send when. See https://github.com/cpan-testers/cpantesters-backend/blob/master/old/mailer/lib/CPAN/Testers/WWW/Reports/Mailer.pm for the current report mailer.