[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)

services/auth/auth.go

@@ -72,10 +72,6 @@ func handleSignIn(resp http.ResponseWriter, req *http.Request, sess SessionStore
 	if err != nil {
 		log.Error(fmt.Sprintf("Error setting session: %v", err))
 	}
-	err = sess.Set("uname", user.Name)
-	if err != nil {
-		log.Error(fmt.Sprintf("Error setting session: %v", err))
-	}
 
 	// Language setting of the user overwrites the one previously set
 	// If the user does not have a locale set, we save the current one.