Add paging to the GetBannedPersons fetch.

[dessalines]

Currently GetBannedPersons fetches all banned users. It would be more performant if paging were added to this endpoint.

Not a huge deal, since this is only available to admins anyway.

[Nutomic]

See #4580 for an example how to implement this.

[dessalines]

Also, we'd like to convert every existing page and limit to cursor pagination, like list_posts . But I'd need @dullbananas expertise on how to do that correctly.