In a future diff we introduce a notificationCollapseKey so that we have access to the SIDEBAR_SOURCE when generating the notif text for the CREATE_SIDEBAR mention.

Ahead of that, this diff updates the notificationTexts code so that it doesn't assert that there is only one message in the collapse result.

Depends on D6867

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"