[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)

routers/web/auth/oauth.go

@@ -1118,8 +1118,7 @@ func handleOAuth2SignIn(ctx *context.Context, source *auth.Source, u *user_model
 	// we can't sign the user in just yet. Instead, redirect them to the 2FA authentication page.
 	if !needs2FA {
 		if err := updateSession(ctx, nil, map[string]any{
-			"uid":   u.ID,
-			"uname": u.Name,
+			"uid": u.ID,
 		}); err != nil {
 			ctx.ServerError("updateSession", err)
 			return