Switch to unified timeline cache via TimelineKinds

This is a fairly large rewrite which unifies our threads, timelines and
profiles. Now all timelines have a MultiSubscriber, and can be added
and removed to columns just like Threads and Profiles.

Signed-off-by: William Casarin <jb55@jb55.com>

crates/notedeck_columns/src/error.rs

@@ -5,6 +5,9 @@ pub enum Error {
     #[error("timeline not found")]
     TimelineNotFound,
 
+    #[error("timeline is missing a subscription")]
+    MissingSubscription,
+
     #[error("load failed")]
     LoadFailed,