[keyserver/lib] Update isInvalidSidebarSource to check messageSpec.canBeSidebarSource
Summary:
Now that existing message specs each have a canBeSidebarSource method, I need to update the function isInvalidSidebarSource to check messageSpec.canBeSidebarSource.
Depends on D9325
Test Plan: Ran flow and manually tested sidebar creation on a variety of message robotext types & text messages
Reviewers: atul, ginsu, ashoat
Reviewed By: ashoat
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D9330