Offer broadcast options based on attendance status

When using the broadcast message form, I believe it would be very helpful to offer the following options for the recipients:

All signups
Users that have attended
Users that have not attended
Users without a recorded attendance

Attached is a patch that does so.

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On a side note, I believe the use of constants should be introduced for attendance status, rather than 1, 0, and NULL.

I had to do:

if (!($signup->attended === '1')) {

Where I should be doing:

if ($signup->attended == SIGNUP_ATTENDANCE_YES) {