This commit adds functionality to import messages from the
Discussions having direct channels as their parent. As we don't
have topics in the PMs, the messages are imported in interleaved
form in the imported direct channels/PMs.
This was completely unsupported earlier and would have resulted in
an error.
While the STREAM_LINK_REGEX and STREAM_TOPIC_LINK_REGEX
identifies the stream and topic mentions in the content
correctly (tested by printing out the matches), the
stream/topic mentions are still not linked to the
corresponding streams/topics for imported messages, as
a `zulip_message` instance is required for linking these
mentions to actual streams/topics (see `StreamPattern`
class in `markdown/__init__.py`) which is not provided
while processing the markdown for imported messages.
This commit allows to import the following from rocketchat:
* All users
* All public/private channels
* All teams and its public/private channels
* All discussion rooms as topics in their parent channel
* All the messages in all the channels
* All private conversations
* Reactions on messages (except for custom emojis)
* Mentions in messages (except @all, @here mentions)
Priyansh Garg