Tracking issue for removing the Redirect validator

[unleashed]

This validator might no longer be necessary. This issue should be closed whenever we find out whether it is or isn't needed as of the latest 3scale versions: it might be the case the admin UI (Porta) does no longer allow people to set these.

This is so because the only OAuth apps users should be creating now should be using OpenID Connect, which handles redirects in proxies way before 3scale gets to authorize/report.

Arising from #280 (comment).

/cc @davidor