* Add Cache-Control to avatar redirects
This does seem to do the trick to make the Avatar redirects cachable
in Chrome.
In Firefox, it does not seem to work, thought and I found no way to
suppress the requests to the original URLs, I even tried setting an
Etag to no avail.
Related discussion in https://github.com/go-gitea/gitea/issues/16964.
Co-authored-by: zeripath <art27@cantab.net>