Fix AttributeError on all pages with posts

The custom defined jinja test "a_post" pointed to an old
location for the Post model that did not reflect the changes
seen in #96.

This commit updates it to use the proper location

src/server.py

@@ -160,7 +160,7 @@ def create_image_client(url, timeout):
     app.ext.environment.globals["format_npf"] = ext_npf_renderer.format_npf
     app.ext.environment.globals["create_poll_callback"] = helpers.create_poll_callback
 
-    app.ext.environment.tests["a_post"] = lambda element : isinstance(element, priviblur_extractor.models.timeline.TimelinePost)
+    app.ext.environment.tests["a_post"] = lambda element : isinstance(element, priviblur_extractor.models.post.Post)
 
 
 @app.listener("main_process_start")