[GITEA] rework long-term authentication

- 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

models/user/user.go

@@ -380,6 +380,11 @@ func (u *User) SetPassword(passwd string) (err error) {
 		return nil
 	}
 
+	// Invalidate all authentication tokens for this user.
+	if err := auth.DeleteAuthTokenByUser(db.DefaultContext, u.ID); err != nil {
+		return err
+	}
+
 	if u.Salt, err = GetUserSalt(); err != nil {
 		return err
 	}