Screen recording does not work with IE11

[mariusandra]

The recorder.js file throws an error when loaded on IE11:

Because Object.assign is not supported

The rrweb docs also say:

rrweb does not support IE11 and below because it uses the MutationObserver API which was supported by these browsers.

Because posthog-js still works with IE11, we should find a way to just disable screen recording when the user is on such an old browser.