Migrate betas.client.factory.js to TypeScript

[liangyuanruo]

The Betas service is used to manage user access to hidden features in the codebase based on the beta flag permissions within the user object. Originally slated for removal, this has found new use with sgID and should be migrated to TS.

[karrui]

the beta flags are also cleared in the DB and not used anymore so all traces should be safe to remove. I will pick up this issue once it's been assigned a priority @liangyuanruo @syan-syan