Add a way to jump forwards and backwards to more activity in the room (seamless navigation¹²).

Fix #46
Follow-up to #71

Summary:

• Changes the "Jump to next activity in room" to actually continue you to the next 100 messages ahead. Previously, it only jumped you to the single next event in the room which meant a lot of backwards overlap each time.
  • Jumping this direction will also start your scroll position at the top of the timeline to continue reading seamlessly ?continue=top
• Adds "Jump to previous activity in room" to the top of the timeline to continue reading the previous part of the conversation.

[1]: See #120

[2]: There is a caveat with seamless here which is also commented on in the code:

XXX: This is flawed in the fact that when we go /messages?dir=b it could backfill messages which will fill up the response before we perfectly connect and continue from the position they were jumping from before. When /messages?dir=f backfills, we won't have this problem anymore because any messages backfilled in the forwards direction would be picked up the same going backwards.

(need forwards fill MSC)

Todo

• Add query parameter that causes the client to start the scroll at the top when jumping forwards so they can continue reading where they left off.
  • This requires Hydrogen changes to not stickToBottom on restoreScrollPosition
  • We might want to refactor to use TimelineView instead of RoomView to pass the options directly and would align with @bwindels suggestion -> Refactor to use TimelineView #117
  • ?scroll=top, ?continue=top
• Stop 404'ing on /jump. Just move to the next day so we can display the no chat messages view
• Add tests
  • 404 when viewing future day
  • Jump back to previous activity
  • Jump forward to next activity
• Update Hydrogen with latest scratch changes