Add P3P to mitigate IE issues

[daaray]

We ran across an issue in Pressweb; @mlavin found a good summary of the issue here https://github.com/mozilla/vinz-clortho/issues/105 and dug up conf he utilizes on another site to resolve the issue.

• add nginx conf to add P3P header to work around IE issues.

[mlavin]

This looks good to me. At best it fixes an issue in IE and at worst is sends an additional meaningless header. :space_invader:

[vrocha]

Should we add this header conditionally or is it not really worth it?

    if ($http_user_agent ~ MSIE ) {
        add_header P3P 'CP="Hello IE"';
    }

[mlavin]

IE 11's user agent string doesn't contain MSIE in the user agent string. See http://blogs.msdn.com/b/ieinternals/archive/2013/09/21/internet-explorer-11-user-agent-string-ua-string-sniffing-compatibility-with-gecko-webkit.aspx it isn't clear if this will be needed for Edge/Spartan (or whatever they are calling it). In general I don't want to try to maintain this user agent matching so I would be a -1 on making this conditional.