[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 picked from commit eff097448b)

[GITEA] rework long-term authentication (squash) add migration

Reminder: the migration is run via integration tests as explained
in the commit "[DB] run all Forgejo migrations in integration tests"

(cherry picked from commit 4accf7443c)
(cherry picked from commit 99d06e344ebc3b50bafb2ac4473dd95f057d1ddc)
(cherry picked from commit d8bc98a8f0)
(cherry picked from commit 6404845df9)
(cherry picked from commit 72bdd4f3b9)
(cherry picked from commit 4b01bb0ce8)
(cherry picked from commit c26ac31816)

modules/setting/security.go

@@ -19,7 +19,6 @@ var (
 	SecretKey                          string
 	InternalToken                      string // internal access token
 	LogInRememberDays                  int
-	CookieUserName                     string
 	CookieRememberName                 string
 	ReverseProxyAuthUser               string
 	ReverseProxyAuthEmail              string
@@ -104,7 +103,6 @@ func loadSecurityFrom(rootCfg ConfigProvider) {
 	sec := rootCfg.Section("security")
 	InstallLock = HasInstallLock(rootCfg)
 	LogInRememberDays = sec.Key("LOGIN_REMEMBER_DAYS").MustInt(7)
-	CookieUserName = sec.Key("COOKIE_USERNAME").MustString("gitea_awesome")
 	SecretKey = loadSecret(sec, "SECRET_KEY_URI", "SECRET_KEY")
 	if SecretKey == "" {
 		// FIXME: https://github.com/go-gitea/gitea/issues/16832