[filebrowser] Add a note to the UI if the file contents are specially rendered

The filebrowser app has the ability to show files uncompressed, and also render Avro files as dictionary objects (HUE-1).

Currently a user can get confused by this automatic behavior and may begin to think that the file isn't compressed or isn't truly an avro file. We should add a small note on such specially rendered views that explicitly tells them what Hue's done there, to avoid this.

The files of interest are apps/filebrowser/src/filebrowser/views.py for the pieces of code that get sent to the UI template, and apps/filebrowser/src/filebrowser/templates/display.mako for the actual UI template where the message needs to be shown and highlighted.