get_or_create_user_profile_from_slack_ids is too expensive

[JnBrymn]

get_or_create_user_profile_from_slack_ids currently goes to Slack for data upon every request. Instead it should memoize or cache the results so that at most it goes back only once per request at most - and maybe even less.

Ideas

• get the user out of the db and only check slack if the update_at is older than some cutoff

[mrshwah]

@JnBrymn I think I improved this using the lru_cache helper in my recent PR. We can probably close once that is merged.