Persist status viewdata as a separate StatusViewDataEntity, and join
this with each status when building from the cached timeline.

This allows the user's viewdata choices to persist after the cached
timeline is cleared during a refresh operation.

Add a repository method to save the status viewdata, and call that
from the viewmodel instead of per-viewdata item methods.

- Periodically prune the StatusViewDataEntity table
- Remove the data when an account is logged out

The previous code would always fetch the latest statuses when the app
restarts, jumping the user to the top of the home timeline. This is
because state.anchorPosition was null in this case.

Fix this by passing the saved initialKey to CachedTimelineRemoteMediator
and using it to construct a page of statuses around the requested
status.

This restores the user's reading position, and ensures that if the
user is at the top of the list they're reading position is not reset
to the second item in the list.

Fixes pachli#41, pachli#42

…position

# Conflicts:
#	app/src/main/java/app/pachli/components/timeline/viewmodel/CachedTimelineRemoteMediator.kt