Add `authenticationManager` to `TalkPageViewModel`

[staykids]

Phabricator: https://phabricator.wikimedia.org/T311637 (partially)

Notes

This adds the shared WMFAuthenticationManager as a passed property on TalkPageViewModel. I went this route over passing the entire MWKDataStore as a dependency just to keep things leaner and more focused. I'm happy to close this though if I'm missing a benefit we'd have for this feature in passing the entire data store as a dependency here or if someone else had plans for how to pass this data along.