Slack bridge: Update Slack bridge with Events API.

Currently we use the Slacks legacy RTM API as the
"listener" from Slack to Zulip. This commit replaces
RTM API for the supported Events API instead.

Fixes zulip#825.

zulip/integrations/bridge_with_slack/run-slack-bridge

@@ -154,7 +154,7 @@ if __name__ == "__main__":
     print("MAKE SURE THE BOT IS SUBSCRIBED TO THE RELEVANT ZULIP STREAM(S) & SLACK CHANNEL(S)!")
 
     # We have to define rtm outside of SlackBridge because the rtm variable is used as a method decorator.
-    rtm = RTMClient(token=config["slack"]["token"])
+    rtm = RTMClient(token=config["slack"]["token"]) // OUTDATED
 
     backoff = zulip.RandomExponentialBackoff(timeout_success_equivalent=300)
     while backoff.keep_going():