dmails: don't default IP addr to 127.0.0.1 in database (fixes #2908).

Bug introduced in 1400f64; that commit changed dmails so that
creator_ip_addr defaulted to CurrentUser.ip_addr like this:

    after_initialize :initialize_attributes, if: :new_record?

    def initialize_attributes
      self.from_id ||= CurrentUser.id
      self.creator_ip_addr ||= CurrentUser.ip_addr
    end

...but creator_ip_addr already defaulted to 127.0.0.1 from the database,
so the ||= assignment didn't work. Remove the database default so we
always default to CurrentUser.ip_addr.

db/migrate/20170302014435_remove_default_ip_addr_from_dmails.rb

@@ -0,0 +1,9 @@
+class RemoveDefaultIpAddrFromDmails < ActiveRecord::Migration
+  def up
+    change_column_default(:dmails, :creator_ip_addr, nil)
+  end
+
+  def down
+    change_column_default(:dmails, :creator_ip_addr, "127.0.0.1")
+  end
+end