Prevent 500 error for unassigned badges

In some cases, badge-saving doesn't go through the normal checks to
ensure data validity. This converts an empty string to None for
birthdates to stop a 500 error from happening upon insertion.
Replace group.new_badge_type

Replaces the new_badge_type property, which was hardcoded to only care
about Guest badges. Groups now had a drop-down for badge type that
affects only new badges.
Show admin notes on group index page

Adds a column to the group index page so notes can be viewed
at-a-glance.
Always display group management link

Moves the "Link for group leader" outside of a sorted_attendees check,
so it will display even for empty groups.
Fix group badge claiming

The form is still ugly, but at least it has all the elements on it.
Merge from ac-staging

Resolve conflicts for PR.
Fix design for register_group_member

Adds a missing form tag, so now register_group_member looks good.
Merge pull request #802 from magfest/dealer_changes

Group fixes for AC