[lib] Block creating sidebar from a message which is already a source of a thread

Test Plan:
Create a sidebar and propote it to a subthread.
Check if it isn't possible to create a sidebar from source message in the subthread on both native and web.
Check if it's possible to open the sidebar from the parent thread.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D999