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
@malandrina also raised this issue and I think its perfectly valid. I don't believe so.
I think there are two options though: 
1) We can split it on the space if we want and save them to our db as `first_name` and `last_name` and send them to intercom as custom attributes
2) We can just send first and last names into intercom as custom attributes.

I'm leaning towards the latter as it keeps our system on a single field as github is.

Thoughts?