No-cache requests

[joanniclaborde]

Fixes #133

Solidus' resource cache is great to speed up pages for general users, but seriously slows down testing and publishing new content. This pull request introduces a new "no-cache mode" to fix this problem. The mode is enabled when the first subdomain of the host is preview (by default, configurable with the no_cache_subdomain option). This means a production site can be hosted and browsed normally on domain.com or abc.domain.com, and its no-cache version will be accessible through preview.domain.com or preview.abc.domain.com. This pull request makes Solidus follow the no-cache request headers, which are sent when a user forces-refresh a page in his browser.

When the Cache-Control: no-cache request is detected, the following changes happen to the request:

• The Cache-Control: no-cache and Pragma: no-cache request HTTP headers are added to all resources. This enables the end-to-end reload of the resources from the proxies and APIs.
• The resources cache is skipped, in order to get fresh data from the APIs. The resources cache is still updated with the fresh data, so normal requests can benefit from it.
• The response HTTP headers are set to disable the caching and indexation of the response: Cache-Control: no-cache, no-store, max-age=0, must-revalidate and X-Robots-Tag: noindex, nofollow.

[joanniclaborde]

Note: I didn't use the "preview mode" name since it already exists.

[joanniclaborde]

Maybe the default subdomain should be no-cache instead of preview...