Store User Names from Github

* Will improve intercom data, ie. allow us to address by name
* Includes a one off rake task for backfilling exisiting user's names
Since we only need the user IDs, I think this is a good use-case for [`pluck`](http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-pluck).
```ruby
User.pluck(:id).each do |user_id|
  JobQueue.push(UserNameJob, user_id)
end
```
It should improve performance, though it might not matter very much in this case. Thoughts?