* Use Slack display names instead of the "name" field
The "name" field is the left-hand side of the email address in many
cases. This information is not otherwise available so we shouldn't
expose it in forum posts.
* Fall back to real_name, add comments
* Store users as a hash
This should avoid a lot of O(n) lookups
* Format user mentions with the correct name also
* Fix the tests (first try)
* Fix the tests (second try); add a test for user mentions
* Fix the tests (third try)
* Empty commit to trigger Travis
* Fix the tests (fourth try)
* Fix the tests (fifth try)
* Change spaces to underscores
* Updates per feedback
