implement useAuthor().roles: []Role

[estebanabaroa]

it's not possible to get a list of all roles in all subplebbits for an author, but the subplebbits cache contains all roles for all known subplebbits, so it's possible to get a list of all known and cached roles. we should not refetch all subplebbits as this is really expensive and knowing the roles is not that critical, but we can use the cache.