After ignored users list is updated, use pagination to refill the cleared timelines

[kevinaboos]

But, there are still a couple of things we need to fix up:

• saving which event ID is currently shown on each timeline, such that we can jump back to displaying that event after the timelines get cleared and then refilled.
• subscribing to an ignored user stream, such that we can receive an update when the current logged-in user (un)ignores someone from a different client that we do not control.
  • See Client::subscribe_to_ignore_user_list_changes(): https://matrix-org.github.io/matrix-rust-sdk/matrix_sdk/struct.Client.html#method.subscribe_to_ignore_user_list_changes

Ideally we also do not want to re-paginate all rooms, just the ones that were already (partially) paginated.