Refactoring to work with Slack's API changes

[k-mahoney]

Slack announced that they are removing the username object from their API and will now only support a mutable display_name object and the full user id - W012A3CDE.

https://api.slack.com/changelog/2017-09-the-one-about-usernames

Currently we track the username in our employee database, which is what we query to associate the GitHub username or other IDs from the initial alarm with the Slack user. We will no longer be able to rely on this.

Some initial thoughts on how to pivot and account for this change...

• Replace the stored Slack username with the full, immutable user id in internal database
• Further improve fallback and error handling for missing username cases

/cc @mapbox/security

[k-mahoney]

Noting that we technically have until September 12, 2018 when Slack drops support for the username object, but are already running into problems with users that have null usernames. Will prioritize updating for Slack's changes, error handling for missing usernames is in place via https://github.com/mapbox/dispatch/pull/78.

[k-mahoney]

In progress in https://github.com/mapbox/dispatch/pull/116

[k-mahoney]

Complete in #116