[lib] Include fact that you were tagged in sidebar creation message

Summary:
In the stack ending with D6874, I made it so that if you were @-mentioned in the OP, you would get a notif that ended with "that tagged you". (Part 1 from this Linear comment.)

This diff completes the stack that makes it so if you were @-mentioned in the message that triggers the creation of the sidebar, you will get a notif that ends with "and tagged you". (Part 2 from this Linear comment.)

Depends on D6937

Test Plan:
Flow, and tested in combination with the next diffs to confirm that getting @-tagged in a message that triggers the creation of a sidebar will update the SIDEBAR_CREATION notif
to be suffixed with "and tagged you"

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6938