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