[keyserver] Convert flag to a boolean

Summary: Instead of returning a boolean we were returning a number. Everything worked correctly, because 0 is falsy. Only recently introduced output validator made the bug evident.

Test Plan: Check if fetching invite links works.

Reviewers: bartek, kamil, inka, michal

Reviewed By: michal

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7941