fix(webClientServer): use relative path in logoutEndpointUrl

[jsjoeio]

Description

This patches the webClientServer to ensure the logoutEndpointUrl is relative to the root.
This ensures the correct URL in case code-server is being served behind a
reverse proxy.

An example of this would be using Caddy to server code-server on
localhost:8082/code/

This fix ensures logging out makes a request to localhost:8082/code/logout instead
of localhost:8082/logout

Video

Before

Screen.Recording.2021-11-23.at.4.01.12.PM.mov

After

Screen.Recording.2021-11-23.at.4.03.14.PM.mov

Testing Plan

I tested against the root and the reverse-proxy.

1. cd vscode && git checkout jsjoeio-fix-csp-reverse-proxy
2. yarn link
3. cd code-server && yarn link code-oss-dev --modules-folder vendor/modules
4. create Caddyfile somewhere:

http://localhost:8082/code/* {
  uri strip_prefix /code
  reverse_proxy 127.0.0.1:8080
}

5. caddy run
6. Navigate to http://localhost:8082/code/ in the browser
7. Open Menu and click Sign out

This PR fixes https://github.com/cdr/code-server/issues/4476

(There is a chance this fixes other reverse proxy issues, but I will test after this is merged).