Messages send to forum threads need to use the ID of the forum for the
webhook but also need to set an additional parameter during the API call
to specify the destination thread.
Receiving messages from threads works by using the thread ID. The forum
ID is not required.
This commit adds an optional channel option "ForumID" which can be used
to supply the forum ID of a thread (= channel). When sending/editing
messages the ForumID will be used for the webhook and the channel ID
will be supplied as the additional parameter "thread_id".
Fixes#1908
3e3090fc53