Recovered `api/v1/sessions_controller.rb` after lost in merge.
Recovered `api/v1/sessions_controller.rb` after lost in merge.

It was accidentally removed in [add5b1a7](https://github.com/concord-consortium/rigse/pull/524/commits/add5b1a72d7e5627c5d6cc66763a4d9969ed156b)

concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse