Disable HTTP caches on preview links (for 2.x)

[chillu]

See silverstripe/silverstripe-framework#8086
See #88

[chillu]

Tested with the following state:

.env

SS_ENVIRONMENT_TYPE="live"

Enabled caching on pages. Removed comments to avoid PHP sessions started.
Running silverstripe/silverstripe-framework#8267 and https://github.com/tractorcow/silverstripe-fluent/issues/440 patches for the same reason.

Result (confirming cache-control flags come through):

curl -I http://cwp2-installer.vagrant/about-us
HTTP/1.1 200 OK
Server: nginx
Date: Fri, 20 Jul 2018 03:44:18 GMT
Content-Type: text/html; charset=utf-8
Connection: keep-alive
vary: X-Requested-With,X-Forwarded-Protocol
cache-control: must-revalidate, max-age=10
expires: Fri, 20 Jul 2018 03:44:28 GMT
etag: "2e67c5bd2f0c456e7cb2b3285eecd741"
x-xss-protection: 1; mode=block

Result on preview link (note the no-cache bits):

curl -I http://cwp2-installer.vagrant/preview/c0432ed6ee045834/0a1413f336a1140b
HTTP/1.1 200 OK
Server: nginx
Date: Fri, 20 Jul 2018 04:09:20 GMT
Content-Type: text/html; charset=utf-8
Connection: keep-alive
Set-Cookie: PHPSESSID=c9vr8p5b2d1cfpntoncl6d8b2c; expires=Fri, 20-Jul-2018 05:09:20 GMT; Max-Age=3600; path=/; HttpOnly
Set-Cookie: PHPSESSID=c9vr8p5b2d1cfpntoncl6d8b2c; expires=Fri, 20-Jul-2018 05:09:20 GMT; Max-Age=3600; path=/; HttpOnly
vary: X-Requested-With,X-Forwarded-Protocol
cache-control: no-cache, no-store, must-revalidate
x-xss-protection: 1; mode=block

[dhensby]

Why not private cache?

[chillu]

Being conservative? Sure, you could treat repeat requests from the same browser as cacheable within this browser, in case something else in the request processing set a max age. But that's pretty low value. The important bit is to ensure there's no caching, and the default way of achieving this is through disableCache(). If you feel strongly about this, feel free to change it to privateCache() (for both 1.x and 2.x PRs)