Add username as custom new relic variable  [MJ-389]

Adds user_id to tracking traces in New Relic. In order to test it you need to:
 (1) Have a newrelic.ini file in the main myjobs directory (I used the directseo one and just made sure to change the app_name on line 36)
 (2) Run the server using NEW_RELIC_CONFIG_FILE=newrelic.ini newrelic-admin run-python manage.py runserver

I'm not sure if/how the settings will need to carry over to newrelic.ini since it's not a tracked file, although I imagine something already exists since we've been tracking stuff on MyJobs.

All tests pass.