[v1.21] [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)

models/forgejo_migrations/v1_20/v3.go

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+
+package forgejo_v1_20 //nolint:revive
+
+import (
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"xorm.io/xorm"
+)
+
+type AuthorizationToken struct {
+	ID              int64  `xorm:"pk autoincr"`
+	UID             int64  `xorm:"INDEX"`
+	LookupKey       string `xorm:"INDEX UNIQUE"`
+	HashedValidator string
+	Expiry          timeutil.TimeStamp
+}
+
+func (AuthorizationToken) TableName() string {
+	return "forgejo_auth_token"
+}
+
+func CreateAuthorizationTokenTable(x *xorm.Engine) error {
+	return x.Sync(new(AuthorizationToken))
+}