Allow to switch the icon to code-server icon

[krassowski]

Currently the VS Code icon is used even when code-server is run. This is problematic because the Visual Studio Code icon is a Microsoft trademark and should not be used to represent products other than VS Code as per https://code.visualstudio.com/brand

Proposed solution:

• ship the code-server icon (https://github.com/coder/code-server/blob/main/src/browser/media/favicon.svg) as well
• allow to switch the icon based on an environment variable config

Any thoughts/counter ideas before I open a PR?