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_ui/src/collab_panel.rs

@@ -940,7 +940,7 @@ impl CollabPanel {
             room.read(cx).local_participant().role == proto::ChannelRole::Admin
         });
 
-        ListItem::new(SharedString::from(user.github_login.clone()))
+        ListItem::new(user.github_login.clone())
             .start_slot(Avatar::new(user.avatar_uri.clone()))
             .child(Label::new(user.github_login.clone()))
             .toggle_state(is_selected)
@@ -2583,7 +2583,7 @@ impl CollabPanel {
     ) -> impl IntoElement {
         let online = contact.online;
         let busy = contact.busy || calling;
-        let github_login = SharedString::from(contact.user.github_login.clone());
+        let github_login = contact.user.github_login.clone();
         let item = ListItem::new(github_login.clone())
             .indent_level(1)
             .indent_step_size(px(20.))
@@ -2662,7 +2662,7 @@ impl CollabPanel {
         is_selected: bool,
         cx: &mut Context<Self>,
     ) -> impl IntoElement {
-        let github_login = SharedString::from(user.github_login.clone());
+        let github_login = user.github_login.clone();
         let user_id = user.id;
         let is_response_pending = self.user_store.read(cx).is_contact_request_pending(user);
         let color = if is_response_pending {