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/ui/src/components.rs

@@ -34,6 +34,7 @@ mod stack;
 mod sticky_items;
 mod tab;
 mod tab_bar;
+mod theme_preview;
 mod toggle;
 mod tooltip;
 
@@ -76,6 +77,7 @@ pub use stack::*;
 pub use sticky_items::*;
 pub use tab::*;
 pub use tab_bar::*;
+pub use theme_preview::*;
 pub use toggle::*;
 pub use tooltip::*;