don't compute etags in static file handlers

etags skip cache timing, which we probably don't want.  If-Modified-Since is good enough for us.

IPython/html/base/handlers.py

@@ -221,6 +221,9 @@ class AuthenticatedFileHandler(IPythonHandler, web.StaticFileHandler):
         
         return web.StaticFileHandler.get(self, path)
     
+    def compute_etag(self):
+        return None
+    
     def validate_absolute_path(self, root, absolute_path):
         """Validate and return the absolute path.
         
@@ -312,6 +315,9 @@ class FileFindHandler(web.StaticFileHandler):
         )
         self.default_filename = default_filename
     
+    def compute_etag(self):
+        return None
+    
     @classmethod
     def get_absolute_path(cls, roots, path):
         """locate a file to serve on our static file search path"""