update vertical and inline layouts for radio choices

rest_framework/templates/rest_framework/horizontal/radio.html

 {% load i18n %}
+{% trans "None" as none_choice %}
 
 <div class="form-group">
     {% if field.label %}
@@ -9,7 +10,7 @@
         {% if field.allow_null or field.allow_blank %}
             <label class="radio-inline">
                 <input type="radio" name="{{ field.name }}" value="" {% if not field.value %}checked{% endif %} />
-                {% trans "None" %}
+                {{ none_choice }}
             </label>
         {% endif %}
         {% for key, text in field.choices.items %}
@@ -23,7 +24,7 @@
             <div class="radio">
                 <label>
                     <input type="radio" name="{{ field.name }}" value="" {% if not field.value %}checked{% endif %} />
-                    {% trans "None" %}
+                    {{ none_choice }}
                 </label>
             </div>
         {% endif %}

rest_framework/templates/rest_framework/inline/radio.html

+{% load i18n %}
+{% trans "None" as none_choice %}
+
 <div class="form-group {% if field.errors %}has-error{% endif %}">
     {% if field.label %}
         <label class="sr-only">{{ field.label }}</label>
     {% endif %}
+    {% if field.allow_null or field.allow_blank %}
+        <div class="radio">
+            <label>
+                <input type="radio" name="{{ field.name }}" value="" {% if not field.value %}checked{% endif %}>
+                {{ none_choice }}
+            </label>
+        </div>
+    {% endif %}
     {% for key, text in field.choices.items %}
         <div class="radio">
             <label>

rest_framework/templates/rest_framework/vertical/radio.html

+{% load i18n %}
+{% trans "None" as none_choice %}
+
 <div class="form-group {% if field.errors %}has-error{% endif %}">
     {% if field.label %}
         <label {% if style.hide_label %}class="sr-only"{% endif %}>{{ field.label }}</label>
     {% endif %}
     {% if style.inline %}
         <div>
+        {% if field.allow_null or field.allow_blank %}
+            <label class="radio-inline">
+                <input type="radio" name="{{ field.name }}" value="" {% if not field.value %}checked{% endif %} />
+                {{ none_choice }}
+            </label>
+        {% endif %}
         {% for key, text in field.choices.items %}
             <label class="radio-inline">
                 <input type="radio" name="{{ field.name }}" value="{{ key }}" {% if key == field.value %}checked{% endif %}>
@@ -12,6 +21,14 @@
         {% endfor %}
         </div>
     {% else %}
+        {% if field.allow_null or field.allow_blank %}
+            <div class="radio">
+                <label>
+                    <input type="radio" name="{{ field.name }}" value="" {% if not field.value %}checked{% endif %} />
+                    {{ none_choice }}
+                </label>
+            </div>
+        {% endif %}
         {% for key, text in field.choices.items %}
             <div class="radio">
                 <label>