[keyserver] modify community fetcher to return channel tag

Summary: rather than a boolean indicating whether the channel tag exists or not, we should return the actual tag. this way, we can check if the channel ID is in the list of channels led by the joining user

Test Plan: fetchCommunityFarcasterChannelTag returned the correct tag for my community tagged with a random farcaster channel ID. it returned null if viewer was not logged in and undefined if the community did not have a channel tag

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13301