Update band checklist notification

Requested via Slack

magwest/templates/emails/guests/band_notification.txt

@@ -0,0 +1,7 @@
+{{ guest.group.name }},
+
+As a guest performing at {{ c.EVENT_NAME_AND_YEAR }}, we need you to complete our Performer Checklist, which includes providing your bio information for our website and filling out information for your unassigned badges.
+
+You can complete every step on the checklist at {{ c.URL_BASE }}/guests/index?id={{ guest.id }}
+
+{{ c.BAND_EMAIL_SIGNATURE }}