[SECURITY] Rework long-term authentication

- This is a 'front-port' of the already existing patch on v1.21 and
v1.20, but applied on top of what Gitea has done to rework the LTA
mechanism. Forgejo will stick with the reworked mechanism by the Forgejo
Security team for the time being. The removal of legacy code (AES-GCM) has been
left out.
- 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 picked from commit e3d6622a63)
(cherry picked from commit fef1a6dac5)

tests/integration/integration_test.go

@@ -17,6 +17,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -299,6 +300,12 @@ func loginUser(t testing.TB, userName string) *TestSession {
 
 func loginUserWithPassword(t testing.TB, userName, password string) *TestSession {
 	t.Helper()
+
+	return loginUserWithPasswordRemember(t, userName, password, false)
+}
+
+func loginUserWithPasswordRemember(t testing.TB, userName, password string, rememberMe bool) *TestSession {
+	t.Helper()
 	req := NewRequest(t, "GET", "/user/login")
 	resp := MakeRequest(t, req, http.StatusOK)
 
@@ -307,6 +314,7 @@ func loginUserWithPassword(t testing.TB, userName, password string) *TestSession
 		"_csrf":     doc.GetCSRF(),
 		"user_name": userName,
 		"password":  password,
+		"remember":  strconv.FormatBool(rememberMe),
 	})
 	resp = MakeRequest(t, req, http.StatusSeeOther)