Fix new thread private behaviour

app/templates/threads/new.html

@@ -35,10 +35,18 @@
 		</div>
 	</div>
 
-	{{ render_checkbox_field(form.private, class_="my-3") }}
-	<p>
-		{{ _("Only you, the package author, and users of Approver rank and above can read private threads.") }}
-	</p>
+	{% if allow_private_change %}
+		{{ render_checkbox_field(form.private, class_="my-3") }}
+	{% elif form.private.data %}
+		<p>
+			Private.
+		</p>
+	{% endif %}
+	{% if allow_private_change or form.private.data %}
+		<p>
+			{{ _("Only you, the package author, and users of Approver rank and above can read private threads.") }}
+		</p>
+	{% endif %}
 
 	{{ render_submit_field(form.submit) }}
 </form>