[keyserver] create invite link based on newly created sidebar

Summary:
This creates an invite link for the newly created sidebar.

Depends on D13782

Test Plan: console logged the invitelink. On my iOS simulator dev build, plugged in the invite link to safari with https://comm.app/invite/<value> and confirmed I successfully joined the community and sidebar with a user originally not in either.

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13783