onboarding ui: Add theme preview tiles and button functionality to basic page (#35413)

This PR polishes and adds functionality to the onboarding UI with a
focus on the basic page. It added theme preview tiles, got the Vim,
telemetry, crash reporting, and sign-in button working.

The theme preview component was moved to the UI crate and it now can
have a click handler on it.

Finally, this commit also changed `client::User.github_login` and
`client::UserStore.by_github_login` to use `SharedStrings` instead of
`Strings`. This change was made because user.github_login was cloned in
several areas including the UI, and was cast to a shared string in some
cases too.

Release Notes:

- N/A

---------

Co-authored-by: Remco Smits <djsmits12@gmail.com>

crates/collab/src/tests.rs

@@ -38,12 +38,12 @@ fn room_participants(room: &Entity<Room>, cx: &mut TestAppContext) -> RoomPartic
         let mut remote = room
             .remote_participants()
             .values()
-            .map(|participant| participant.user.github_login.clone())
+            .map(|participant| participant.user.github_login.clone().to_string())
             .collect::<Vec<_>>();
         let mut pending = room
             .pending_participants()
             .iter()
-            .map(|user| user.github_login.clone())
+            .map(|user| user.github_login.clone().to_string())
             .collect::<Vec<_>>();
         remote.sort();
         pending.sort();