implement threads in slack import tool
implement threads in slack import tool.
right now, threaded messages lose their context when imported. they just become normal messages and not replies to other messages.
in slack, replies are standalone objects. it works via a `thread_ts` (thread timestamp) attribute. `thread_ts` refers to the `ts` (timestamp) of the parent post. if a post has `thread_ts` == `ts`, then it is the beginning of a thread.
import all posts except for ones where `thread_ts` != `ts` (as these are replies to other posts). once that's done, go back and import the replies since their parents will have already been imported.
Meanwhile it would be good to add a comment to the Limitations section of the documentation, that the the import of threads is currently not possible: