Slack <--> Zulip bridge

This is a bridge between Slack and Zulip.

Usage

Create a generic Zulip bot, with a full name like Slack Bot.
(Important) Subscribe the bot user to the Zulip stream you'd like to bridge your Slack channel into.
In the zulip section of the configuration file, enter the bot's zuliprc details (email, api_key, and site).
In the same section, also enter the Zulip stream and topic.

Make sure Websocket isn't blocked in the computer where you run this bridge. Test it at https://www.websocket.org/echo.html.
Go to https://api.slack.com/apps?new_classic_app=1 and create a new classic app (note: must be a classic app). Choose a bot name that will be put into bridge_with_slack_config.py, e.g. "zulip_mirror". In the process of doing this, you need to add oauth token scope. Simply choose bot. Slack will say that this is a legacy scope, but we still need to use it anyway. The reason why we need the legacy scope is because otherwise the RTM API wouldn't work. We might remove the RTM API usage in newer version of this bot. Make sure to install the app to the workspace. When successful, you should see a token that starts with "xoxb-...". There is also a token that starts with "xoxp-...", we need the "xoxb-..." one.
Go to "App Home", click the button "Add Legacy Bot User".
(Important) Make sure the bot is subscribed to the channel. You can do this by typing e.g. /invite @zulip_mirror in the relevant channel.
In the slack section of the Zulip-Slack bridge configuration file, enter the bot name (e.g. "zulip_mirror") and token, and the channel ID (note: must be ID, not name).

Run python3 run-slack-bridge