Allow preserving HTTP protocol on redirects for legacy sites

Required because not all legacy sites support HTTPS connections

After deploying, we need to run the following migration manually to adjust the already imported entries:

UPDATE redirections
       SET target_url = regexp_replace(target_url, '^https:', '')
       WHERE creation_timestamp < date('2023-03-29 08:00:00')
       AND slug NOT LIKE 'go/%'
       AND target_url LIKE 'https:%';