Prevent browser caching during auth flow (#453)

* Prevent browser caching during auth flow

* simplify no-cache logic, add tests and update changelog

* checking noCacheHeaders does not exists in response headers from upstream

* remove unnecessary codes

* add no-cache headers in SignInPage and OAuthStart for proxy mode

https://github.com/oauth2-proxy/oauth2-proxy/pull/453#discussion_r405072222