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/integration_tests.rs

@@ -1881,7 +1881,7 @@ async fn test_active_call_events(
         vec![room::Event::RemoteProjectShared {
             owner: Arc::new(User {
                 id: client_a.user_id().unwrap(),
-                github_login: "user_a".to_string(),
+                github_login: "user_a".into(),
                 avatar_uri: "avatar_a".into(),
                 name: None,
             }),
@@ -1900,7 +1900,7 @@ async fn test_active_call_events(
         vec![room::Event::RemoteProjectShared {
             owner: Arc::new(User {
                 id: client_b.user_id().unwrap(),
-                github_login: "user_b".to_string(),
+                github_login: "user_b".into(),
                 avatar_uri: "avatar_b".into(),
                 name: None,
             }),
@@ -6079,7 +6079,7 @@ async fn test_contacts(
                 .iter()
                 .map(|contact| {
                     (
-                        contact.user.github_login.clone(),
+                        contact.user.github_login.clone().to_string(),
                         if contact.online { "online" } else { "offline" },
                         if contact.busy { "busy" } else { "free" },
                     )