Add support for multiple registry types

[auryn-macmillan]

#473 details and addresses some of the issues in the app if a deployment is using something other than the optimistic recipient registry. Ideally, we should generalized the app so that it supports the optimistic, simple, and kleros registries. Would should consolidate this with the effort to use the metadata registry.

We should:

• replace _metadata with _metadataId in each of the registries
• pull all recipient metadata from the metadata registry
• remove the /#/join route and any copy, links, and CTAs to it when the simple registry is in use.
• build native support for the kleros registry, with the assumption that all metadata should be stored in the metadata registry. (should resolve Insufficient recipient metadata for Kleros registry recipients #480)