[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)

models/forgejo_migrations/migrate.go

@@ -41,6 +41,8 @@ var migrations = []*Migration{
 	NewMigration("Add Forgejo Blocked Users table", forgejo_v1_20.AddForgejoBlockedUser),
 	// v1 -> v2
 	NewMigration("create the forgejo_sem_ver table", forgejo_v1_20.CreateSemVerTable),
+	// v2 -> v3
+	NewMigration("create the forgejo_auth_token table", forgejo_v1_20.CreateAuthorizationTokenTable),
 }
 
 // GetCurrentDBVersion returns the current Forgejo database version.