Add ability to filter instances by Registration

[DraconicNEO]

I think it would be good for people to be able to filter instances on the list by whether they have open registration, application, or closed. Should be feasible since the instances listed are tagged according to their registration status.

This would be a good feature since it would better help people pick out instances, some people might want to join ones that are open registration, while others might want to avoid them and only go for application based signups. Also is helpful to see which ones are closed currently.

[dessalines]

Currently we're completely filtering out ones with open registrations, since they're a major spam attack vector.

[DraconicNEO]

Currently we're completely filtering out ones with open registrations, since they're a major spam attack vector.

Still think that it would be a good idea for people to be able to filter by that, since it can still be used to exclude ones with closed registration already on the list. Plus in the future when we have better spam mitigation it'll make sense for instances to have open registration, because you know easy onboarding process for new users and not having to review every sign up manually.