[keyserver] Validate community ID corresponds to a community root in role-creator

Summary:
This diff adds a validation check to make sure the community ID passed in to the server corresponds to a community root (where community roles are valid)

This was some feedback from @ashoat to include this check, and I should have originally done it when writing this code

Depends on D9897

Test Plan: Made sure that it works for valid thread types, and that an error was thrown if a non-community ID was passed in

Reviewers: atul, ginsu, ashoat

Reviewed By: ashoat

Subscribers: tomek, ashoat

Differential Revision: https://phab.comm.dev/D9900