Mention ms-fe servers need to be XFF trusted by MW

Swift is originating rendering (imagescaling) requests by mean of our
own rewrite/404 handler middleware and is copying the originating XFF
header to its own request. Hence, add Swift frontends to the "squid"
server list, which essentially is the "trust XFF headers" list.
This has always been broken, but we were never doing anything with those
IPs until Ie95fdf, which added a per IP rate limit to image scaling
requests. That limit is was essentially applied per Swift frontend IP
and breaking thumb generation for many users.

Bug: 64622
Change-Id: Ica9086dcd32428f7229a4e48e3f210ba0825fc18