[lib] Collapse CreateSidebar and SidebarSource notifs together

Summary:
Before this diff, SidebarSource did not generate any notifs, and CreateSidebar did.

After this diff, technically SidebarSource generates a notif... but not really. Since SidebarSource and CreateSidebar get collapsed into each other, and CreateSidebar is the more recent message, AND the two messages are always generated together on the keyserver... the result is that they only ever generate one notif for the both of them.

The only reason for this is that I want to have access to the SidebarSource message in CreateSidebar's notificationTexts function so that I can check it for @-mentions.

Depends on D6868

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/D6869