fix: use nonce to load chat webview app

Summmary

We have nonce value for the webview but we never use it. So we receive error

Access to script at 'https://gitlab.com/assets/webpack/gitlab-vscode/0.0.1-dev-20230802205337/vscode/extensions/gitlab-vscode-extension/webviews/gitlab_duo_chat/assets/app.js' from origin 'https://0s9miaaag7of9bkkd6p65manuq5gp1cvmo3m4ol1ssrk0kui575u.cdn.web-ide.gitlab-static.net' has been blocked by CORS policy

Also we expand webview wait time to 2 seconds since webview loading is slower in webIDE.