explicitly track user list sync, preference for which users' presence to subscribe

[aoberoi]

Description

in the current implementation, SlackClient#loadUsers() will typically be called twice each time Hubot starts up. the first time, immediately. the second time, after the brain's storage connection is completed and the 'loaded' event is fired. this is wasteful.

the second problem is that subscribing to user presence is tied to the brain 'loaded' event, which seems incorrect. will this ever fire if the brain doesn't have any backing storage (just using memory)? will it only subscribe to a partial list of users (those who were in the brain when it loaded but not those who are modified after SlackClient#loadUsers() completes)?

with the introduction of the Bluebird dependency, it seems like we can use Promises to create a better solution. here's some pseudo-code.

# in SlackBot constructor add `subscribeAllUserPresence` boolean option default to true

# in SlackBot#run()
loadUsers = new Promise (resolve, reject) =>
  @client.loadUsers (error, res) =>
    if (error)
      return reject(error)
    resolve(res)

brainHasStorage = true # no idea how to get this value
initialBrainLoad = not brainHasStorage ? Promise.resolve() : new Promise (resolve, reject) =>
  loaded = false
  @robot.brain.on 'loaded', () ->
    if not loaded
      loaded = true
      resolve()

Promise.all([loadUsers, initialBrainLoad])
  .then ([res]) =>
    @usersLoaded(null, res)
    @presenceSub()
  .catch console.error # TODO

# in SlackBot#presenceSub()
if @options.subscribeAllUserPresence
  # ... current implementation here

# in SlackBot#updateUserInBrain()
if @options.subscribeAllUserPresence && userNotInBrain
  # add a new presence subscription for a single user

should we be handling team_joinevents too?

should we centralize the two places where @robot.brain.userForId are called?

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