[keyserver] Prevent creation of sidebars from pinned message robotext
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
Differential Revision: https://phab.comm.dev/D9232