refactor: Simplify *Timeline{ViewModel,Repository}

[nikclayton]

Remove *ViewModel.getStatuses; instead, trigger the fetch by collecting from *ViewModel.statuses, which is flatmapped from a flow that contains the PachliAccountId to use.

This made it possible to remove the AccountManager.activeAccountFlow references.

The *TimelineRepository implementations are modified to only need a PachliAccountId instead of a full account, which simplifies the code further.