Improve /integrations page load time.

Prior to this commit, 7 megabytes of images (through 253 individual requests)
were heavily slowing down the initial load. With this commit, we load only the
logos (60 or so images).

Documentation and images for the individual integration sub-pages is requested
separately using the /integrations/doc/ endpoint, which returns HTML.

zerver/lib/integrations.py

@@ -93,12 +93,6 @@ class Integration(object):
         # type: (Dict[Any, Any]) -> None
         self.doc_context = context
 
-    @property
-    def help_content(self):
-        # type: () -> Text
-        doc_context = self.doc_context or {}
-        return render_markdown_path(self.doc, doc_context)
-
 class EmailIntegration(Integration):
     def is_enabled(self):
         # type: () -> bool