Fix price bump warning display

The price bump warning now only displays if the attendee's badge
actually costs anything, so now group members claiming a new badge won't
see it.
Fix phone display in admin form

The phone number had the wrong column name. Fixed so it actually
displays the group leader's phone number on the group admin page.
Allow dealers to add badges

Dealers can now add badges to their group based on the number of tables
they have
Rewrite dealer confirmation page

Changes the text on the dealer confirmation page to outline the
registration process better.
Minor text changes

Adds some special-casing for dealers so the dealer pages stop referring
to "group members"
Improve and fix dealer badge properties

Improves dealer_max_badges to be more succinct. Fixes min_badges_addable
to also work for dealers without the "can_add" property checked.
Make amount_unpaid 0 if it's negative

Python considers negative numbers to be truthy, so it was denying
dealers a chance to add more badges if they'd paid MORE than they owed.
Since amount_unpaid doesn't make sense as a negative number, I made it
always return 0 if it's negative.
Add group leader kick-in to group charge

Allows dealers to pay for their badge upgrade while paying for the
group.
Update dealer confirmation page text

Since the group leader's kick-in level is now included in group
payments, the text has been changed to reflect this.
Merge pull request #862 from magfest/fix_dealer_reg

Dealer reg fixes based on feedback