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>
