chatBlocks follow-ups

Follow-ups to https://github.com/nextcloud/spreed/pull/10084:

Link

• 🚧 lastMessage can be made a separate chatBlock, if not present in initial context
• 🐛 'Loading new messages ...' is shown too often (id mismatch?)
• 🐛 In sidebar, there is 'No messages' shown before loader

TODO:

• 🐛 threadBlocks are required to properly keep chat context: https://github.com/nextcloud/spreed/pull/15616
• 🚧 unifying onRouteChange and handleStartGettingMessagesPreconditions or extracting common logic later.
  At the moment:
  • handleStartGettingMessagesPreconditions checks if user 'canGetMessages';
  • onRouteChange triggers only afterwards under separate conditions, assuming the check above is true
• 👀 contextMessageId can still be 0 as per previous unchanged logic. Check if valid
• 👀 compact view doesn't have lastMessage to rely on
  • Compact view changed to still poll with lastMessage
• 👀 federated conversations aren't tested and missing lastMessageId