[lib] Make CreateSidebar notifs say when the OP tags the notif recipient

Summary:
Typically you would only receive a CreateSidebar notif if you are the author of the OP. However, some earlier diffs introduced a new scenario: if you are tagged in the OP, you will be auto-added to the sidebar at creation time, and as such will receive the CreateSidebar notif.

To make sure the notif recipient isn't confused by the notif, this diff adds some text to the notification to clarify this.

Depends on D6873

Test Plan: I confirmed that a sidebar started in response to a text message that @-mentions somebody will result in the @-mentioned user receiving a custom notif that ends with "that tagged you"

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6874