[keyserver] Check SIDEBAR_SOURCE's source message for @-mention notif override

Summary:
If somebody starts a sidebar from a message that has an @-mention, we want to override the @-mentioned user's notifs for the sidebar creation.

Depends on D6866

Test Plan: In combination with the rest of the stack, 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/D6867