Enhanced auth token / remember me (#27606)

Closes #27455

> The mechanism responsible for long-term authentication (the 'remember
me' cookie) uses a weak construction technique. It will hash the user's
hashed password and the rands value; it will then call the secure cookie
code, which will encrypt the user's name with the computed hash. If one
were able to dump the database, they could extract those two values to
rebuild that cookie and impersonate a user. That vulnerability exists
from the date the dump was obtained until a user changed their password.
> 
> To fix this security issue, the cookie could be created and verified
using a different technique such as the one explained at
https://paragonie.com/blog/2015/04/secure-authentication-php-with-long-term-persistence#secure-remember-me-cookies.

The PR removes the now obsolete setting `COOKIE_USERNAME`.

tests/integration/signin_test.go

@@ -5,11 +5,13 @@ package integration
 
 import (
 	"net/http"
+	"net/url"
 	"strings"
 	"testing"
 
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/translation"
 	"code.gitea.io/gitea/tests"
 
@@ -57,3 +59,37 @@ func TestSignin(t *testing.T) {
 		testLoginFailed(t, s.username, s.password, s.message)
 	}
 }
+
+func TestSigninWithRememberMe(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	baseURL, _ := url.Parse(setting.AppURL)
+
+	session := emptyTestSession(t)
+	req := NewRequestWithValues(t, "POST", "/user/login", map[string]string{
+		"_csrf":     GetCSRF(t, session, "/user/login"),
+		"user_name": user.Name,
+		"password":  userPassword,
+		"remember":  "on",
+	})
+	session.MakeRequest(t, req, http.StatusSeeOther)
+
+	c := session.GetCookie(setting.CookieRememberName)
+	assert.NotNil(t, c)
+
+	session = emptyTestSession(t)
+
+	// Without session the settings page should not be reachable
+	req = NewRequest(t, "GET", "/user/settings")
+	session.MakeRequest(t, req, http.StatusSeeOther)
+
+	req = NewRequest(t, "GET", "/user/login")
+	// Set the remember me cookie for the login GET request
+	session.jar.SetCookies(baseURL, []*http.Cookie{c})
+	session.MakeRequest(t, req, http.StatusSeeOther)
+
+	// With session the settings page should be reachable
+	req = NewRequest(t, "GET", "/user/settings")
+	session.MakeRequest(t, req, http.StatusOK)
+}