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/git_ui/src/git_panel.rs

@@ -2416,7 +2416,7 @@ impl GitPanel {
                     .committer_name
                     .clone()
                     .or_else(|| participant.user.name.clone())
-                    .unwrap_or_else(|| participant.user.github_login.clone());
+                    .unwrap_or_else(|| participant.user.github_login.clone().to_string());
                 new_co_authors.push((name.clone(), email.clone()))
             }
         }
@@ -2436,7 +2436,7 @@ impl GitPanel {
             .name
             .clone()
             .or_else(|| user.name.clone())
-            .unwrap_or_else(|| user.github_login.clone());
+            .unwrap_or_else(|| user.github_login.clone().to_string());
         Some((name, email))
     }