Handle threaded messages

[arjunattam]

Receiving threaded messages:

• Case 1: thread broadcast (also sent to channel) - shows up as a separate message in the channel.history response, and thread_ts is equivalent to parent message ts. Also has is_thread_broadcast set to true in the message object
• Case 2: thread replies - show up as replies in the parent message object, but without the reply text - retrieving them is a separate API call

https://api.slack.com/docs/message-threading#finding_message_threads_in_the_wild

Sending threads: haven't investigated, but the same link has a section on this

[arjunattam]

Scenarios to be covered

• [x] real-time: incoming thread-only messages should be updated in the thread list
• [x] real-time: incoming broadcast messages should be updated in thread list and main channel view
• [ ] real-time: incoming broadcast messages should have a way to open the parent thread
• [ ] real-time: if parent message of incoming is not in the fetched history, we will need to fetch more history till we fetch it?
• [x] sending: how to reply to a thread?
• [ ] sending: how to send a broadcast reply to thread?
• [x] historical: show number of replies on messages that have them
• [x] historical: click to open replies, with same formatting variations as channel messages
• [ ] marking read: evaluate the subscribed and unread_count keys in thread parent object. should that be added separately to the channel unread count?
• [ ] unread threads must appear differently than channels - maybe have a separate status bar item? or a different quick pick behavior