[server] Don't allow admins to create child threads of sidebars
[server] Don't allow admins to create child threads of sidebars

Summary: Even if you're an admin, you still shouldn't have `CREATE_SUBTHREADS` or `CREATE_SIDEBARS` on all descendants. This diff restricts it to top-level (ie. non-sidebar) descendants only.

Test Plan: Will be tested in combination with following diffs

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1205

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal