[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)

models/user/user.go

@@ -386,6 +386,11 @@ func (u *User) SetPassword(passwd string) (err error) {
 		return nil
 	}
 
+	// Invalidate all authentication tokens for this user.
+	if err := auth.DeleteAuthTokenByUser(db.DefaultContext, u.ID); err != nil {
+		return err
+	}
+
 	if u.Salt, err = GetUserSalt(); err != nil {
 		return err
 	}