- The current architecture is inherently insecure, because you can
construct the 'secret' cookie value with values that are available in
the database. Thus provides zero protection when a database is
dumped/leaked.
- This patch implements a new architecture that's inspired from: [Paragonie Initiative](https://paragonie.com/blog/2015/04/secure-authentication-php-with-long-term-persistence#secure-remember-me-cookies).
- Integration testing is added to ensure the new mechanism works.
- Removes a setting, because it's not used anymore.
(cherry-pick from eff097448b)
Conflicts:
modules/context/context_cookie.go
trivial context conflicts
routers/web/web.go
ctx.GetSiteCookie(setting.CookieRememberName) moved from services/auth/middleware.go
