[keyserver] reply on farcaster with invite link

Summary:
Putting this diff for review. Currently debugging our invalid signerUUID. This diff replies on farcaster with the invite link

Depends on D13784

Test Plan: Tested with my local iOS simulator comm app. I grabbed the invite link posted to warpcast and pasted it to my safari. Was able to successfully join new communities and/or sidebars

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13785