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>

Cargo.lock

@@ -10923,6 +10923,7 @@ name = "onboarding"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "client",
  "command_palette_hooks",
  "db",
  "editor",
@@ -10937,6 +10938,7 @@ dependencies = [
  "theme",
  "ui",
  "util",
+ "vim_mode_setting",
  "workspace",
  "workspace-hack",
  "zed_actions",
@@ -18594,7 +18596,6 @@ dependencies = [
  "serde",
  "settings",
  "telemetry",
- "theme",
  "ui",
  "util",
  "vim_mode_setting",