Remove extraneous > in Attendance Reminder mailer

An extra '>' character in the URL was causing issues with the registration mail
Consolidate Attendance Mail Callbacks, DRYness

Moved sending of Attendance registration mail from within the TT controller into
the `queue_mais` method on the after_create hook of the Attendance. Eventually
we're likely to want to move all this behaviour out into service objects, but
it makes it easy to track down the source of mails for now.
Switch from maildown gem to markerb

After re-evaluating the two different options, markerb was better maintained and
had more documentation. The file extension isn't as nice, but it's less hacky
and doesn't suffer the wacky mimepart issue that was causing our triplicate email
problem.

Fixes #169
Merge pull request #171 from TeaWithStrangers/mailer-triplicate-fix

Mailer Triplicate Fix