Reimplement generateRandomString using browser crypto module

Summary:
generateRandomString is currently implemented using a polyfill of the Node.js crypto library, but Webpack 5 no longer includes this polyfill by default.

We could configure the polyfill manually, but instead I figured we could save some bundle size and just reimplement this using the browser-native crypto module, which has a getRandomValues function.

Test Plan: console.log'd the results of calling generateDeviceID in web

Reviewers: inka, atul, tomek

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6697