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