Fix incorrect lock warning for new wiki pages

Fixes #625

src/plainui/jinja2/plainui/static_page_edit.html.j2

@@ -19,7 +19,7 @@
   </nav>
 
   <article class="pb-11">
-    {% if not lock_id %}
+    {% if page_exists and not lock_id %}
       {% call alert.warning() %}
         <strong>{{ _("This page is currently being edited by another user, please try again in a moment.") }}</strong>
       {% endcall %}

src/plainui/views/static_pages.py

@@ -210,6 +210,7 @@ class StaticPageEditView(ConferenceRequiredMixin, TemplateView):
             {
                 'page': static_page,
                 'conf': self.conf,
+                'page_exists': page_exists,
                 'page_slug': page_slug,
                 'static_page': static_page,
                 'writeable': writeable,