slack: Fetch workspace users from /users.list in the correct manner.

1. Fetching from the `/users.list` endpoint is supposed to use
   pagination. Slack will return at most 1000 results in a single
   request. This means that our Slack import system hasn't worked
   properly for workspaces with more than 1000 users. Users after the
   first 1000 would be considered by our tool as mirror dummies and thus
   created with is_active=False,is_mirror_dummy=True.
   Ref https://api.slack.com/methods/users.list

2. Workspaces with a lot of users, and therefore requiring the use of
   paginated requests to fetch them all, might also get us to run into
   Slack's rate limits, since we'll be doing repeating requests to the
   endpoint.
   Therefore, the API fetch needs to also handle rate limiting errors
   correctly.
   Per, https://api.slack.com/apis/rate-limits#headers, we can just read
   the retry-after header from the rsponse and wait the indicated number
   of seconds before repeating the requests. This is an easy approach to
   implement, so that's what we go with here.

zerver/webhooks/slack/view.py

@@ -48,6 +48,8 @@ def get_slack_channel_name(channel_id: str, token: str) -> str:
     slack_channel_data = get_slack_api_data(
         "https://slack.com/api/conversations.info",
         get_param="channel",
+        # Sleeping is not permitted from webhook code.
+        raise_if_rate_limited=True,
         token=token,
         channel=channel_id,
     )
@@ -56,7 +58,12 @@ def get_slack_channel_name(channel_id: str, token: str) -> str:
 
 def get_slack_sender_name(user_id: str, token: str) -> str:
     slack_user_data = get_slack_api_data(
-        "https://slack.com/api/users.info", get_param="user", token=token, user=user_id
+        "https://slack.com/api/users.info",
+        get_param="user",
+        # Sleeping is not permitted from webhook code.
+        raise_if_rate_limited=True,
+        token=token,
+        user=user_id,
     )
     return slack_user_data["name"]