Sending User Staff Status to Segment

Staff status determined via email regex.

@rocha @dsjen
My suggestion is to not try to use the word `staff` in the naming of the variables related to segment.io

Like I said earlier, the term staff is very overloaded in our domain (there is the django staff flag, the course staff and edx employees). Instead use something like `IGNORE_FROM_SEGMENT` and make all related variables follow a similar convention.