Given an event, figure out the thread ID we should use for it in a receipt.
This will either be "main", or event.threadRootId. For the thread root, or
e.g. reactions to the thread root, this will be main. For events inside the
thread, or e.g. reactions to them, this will be event.threadRootId.
Given an event, figure out the thread ID we should use for it in a receipt.
This will either be "main", or event.threadRootId. For the thread root, or e.g. reactions to the thread root, this will be main. For events inside the thread, or e.g. reactions to them, this will be event.threadRootId.
(Exported for test.)