Simplify model fetching observables - part 1 of N

[pkukielka]

Changes

Main change is in the structure of observables in the sync.ts method syncModels. Previously it was possible to get multiple updates of auth/config/clientConfig/etc and various promises could be at different stage of updates. E.g. userPreferences could be updated based on older version of config, and remoteModels based on newer version. Also config and auth should always go in sync, otherwise we can get them for two different endpoints.

In general this PR aims to make model syncing more predictable and less error prone.

There will be more PRs following this one.

Test plan

N/A - covered by automated tests