Fix the markdown parsing order

Switch markdown() and linkify() so that the image URLs do'nt
turn into HTML tags.

src/acanban/__init__.py

@@ -91,7 +91,7 @@ async def test_app(self) -> AsyncIterator[Acanban]:
 
 @app.template_filter('markdown')
 def as_markdown(text: str) -> str:
-    html = clean(markdown(linkify(text)), markdown_tags, markdown_attrs)
+    html = clean(linkify(markdown(text)), markdown_tags, markdown_attrs)
     return html