Whenever we create a chat message that is `in_reply_to` another
message, we want to lazily populate the thread record for the
message chain.
If there is no thread yet for the root message in the reply chain,
we create a new thread with the appropriate details, and use that
thread ID for every message in the chain that does not yet have
a thread ID.
* Root message (ID 1) - no thread ID
* Message (ID 2, in_reply_to 1) - no thread ID
* When I as a user create a message in reply to ID 2, we create a thread and apply it to ID 1, ID 2, and the new message
If there is a thread for the root message in the reply chain, we
do not create one, and use the thread ID for the newly created chat
message.
* Root message (ID 1) - thread ID 700
* Message (ID 2, in_reply_to 1) - thread ID 700
* When I as a user create a message in reply to ID 2, we use the existing thread ID 700 for the new message
We also support passing in the `thread_id` to `ChatMessageCreator`,
which will be used when replying to a message that is already part of
a thread, and we validate whether that `thread_id` is okay in the context
of the channel and also the reply chain.
This work is always done, regardless of channel `thread_enabled` settings
or the `enable_experimental_chat_threaded_discussions` site setting.
This commit does not include a large data migration to backfill threads for
all existing reply chains, its unnecessary to do this so early in the project,
we can do this later if necessary.
This commit also includes thread considerations in the `MessageMover` class:
* If the original message and N other messages of a thread is moved,
the remaining messages in the thread have a new thread created in
the old channel and are moved to it.
* The reply chain is not preserved for moved messages, so new threads are
not created in the destination channel.
In addition to this, I added a fix to also clear the `in_reply_to_id` of messages
in the old channel which are moved out of that channel for data cleanliness.
9a45b59fb5