[lib] Include users mentioned in source message in sidebar
Summary:
This addresses ENG-3032.
Depends on D6762
Test Plan:
- Log the list of members at the end of `createPendingSidebar
- Tested in combination with the following diffs, where I show screenshots of a pending sidebar with the @-mentions bolded
Reviewers: atul
Reviewed By: atul
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D6763