Enable room history to be shown immediately upon first opening a room

Move the logic for processing background timeline updates to a
separate function, such that we can run that procedure the very first time
a room is opened. If any updates were processed, we immediately redraw
the timeline, which removes all visible latency that previously existed,
as previously, a timeline wouldn't be redrawn until the first pagination
response was received.

There is one exception to this, which is when a timeline is cleared.
It still gets drawn properly, but since it's been cleared, there is nothing
that actually gets drawn.
This problem requires a completely separate solution, e.g., not actually
setting the existing `TimelineUiState::items` field to the `new_items` vector
from the TimelineUpdate until another (or several more) pagination responses
have been received and the timeline has been sufficiently backfilled.