Change ip_type to use radio buttons

Three models (Ip, Network, and Range) have an "ip_type" field that stores whether some IP address is v4 or v6. This pull request exposes the field as a pair of radio buttons (rather than a drop-down list) in the four forms that use it (AddressRecordForm, AddressRecordFQDNForm, NetworkForm, and RangeForm). It also eliminates the useless and invalid "--------" choice by making ip_type default to IPv4.

(The spelling/grammar corrections weren't originally supposed to be in this pull request, but I don't feel like rebasing, and anyway they're purely cosmetic.)