CLC-3460 Acting-as a user updates their first_login_at field

https://jira.ets.berkeley.edu/jira/browse/CLC-3460

Created the acting_as? method in user_api_controller and commented the places I would plan to use it

Used integration testing for the three possible endpoins that would cause this error

/api/my/record_first_login
- you don't want the admin user to record a first log for a "viewed as" user that does not exist in the database

/api/my/status
- you don't want the admin user to record a visit that's wasn't really made by the "viewed as" user

/api/my/opt_out'
- you don't want the admin user to delete an existing "viewed as" user's data row