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.
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