Handle user slack info change

[hgg]

Describe the bug
When a user changes their info, Corker's local cache should be updated accordingly. If, for instance, a user changes their handle and we don't update our version, any mention to the user will fail.

Expected behavior
We should update our local cache of user info, maintaining the correct functionality of the system.

Additional context
We can use the user_change event from Slack API to update Corker's user info.

[frm]

@Hugainz can I assign you to this?

[hgg]

@justmendes done did it myself 😛