columns: move from Cow<'static, str> to ColumnTitle<'a>

This further deliminates our column titles to those that are simple,
and to those that require additional information from the database.

This allows us to avoid creating many transactions pointlessly if we
don't need to.

Changelog-Changed: Show usernames in user columns

crates/notedeck_columns/src/timeline/mod.rs

@@ -26,7 +26,7 @@ use tracing::{debug, error, info, warn};
 pub mod kind;
 pub mod route;
 
-pub use kind::{PubkeySource, TimelineKind};
+pub use kind::{ColumnTitle, PubkeySource, TimelineKind};
 pub use route::TimelineRoute;
 
 #[derive(Debug, Hash, Copy, Clone, Eq, PartialEq)]