Add Keyboard Lock API support (Chromium 68+ only)

[foresee-io]

This enables web browser to capture key combos, e.g., Alt+F4, Alt+Tab, and send it to web page.

Note: Fullscreen via web browser menu will invalidate this API support. Use full screen button inside the web page instead.

• https://developer.chrome.com/docs/capabilities/web-apis/keyboard-lock
• https://developer.mozilla.org/en-US/docs/Web/API/Navigator/keyboard

[mmcclaskey]

thank you for the pull request, I've entered an internal ticket and tagged for version 1.17 of Kasm Workspaces, so we will be reviewing and doing something with this soon. This will require a lot of testing across hardware, OS, and browser combinations, also languages and keyboard layouts, in order to determine if this is something we can just blindly do without giving the user the ability to disable or putting more caveats in determining when it is enabled or disabled.