Process Slack users.json

[louh]

via #3:

Process users.json:

• Each user is an object in an array. This means looking up any user needs to iterate over the array and check the id property; this could get expensive. If we rewrite the data structure so that the id is a key lookup, matching users to the transcript gets easier to do.
• Surface the useful information: first name and last name is the most important right now. Slack does not seem to use the same properties for everyone (it depends on what data the user provided) so we need to do some normalization.
• Not all information is relevant and it may be dropped.