[keyserver] Prevent creation of sidebars from pinned message robotext

Summary:
These sidebars are causing our keyserver to completely crash and the app to break (ENG-4849). This diff prevents the creation of such sidebars on the keyserver. Preventing the sidebar creation button from appearing in the UI on native and web will be handled separately.

Depends on D9231

Test Plan: Try creating a sidebar from a pinned message in my local environment and confirm that it now errors out

Reviewers: atul, rohan, inka

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9232