[#61733120] Use strong parameters in sessions_controller

To easily use strong parameters
(http://edgeapi.rubyonrails.org/classes/ActionController/StrongParameters.html)
in the sessions controller, the new session form now posts a sessions
object.

(Approach taken from http://edgeapi.rubyonrails.org/classes/ActionController/StrongParameters.html)
Merge pull request #3 from dannyaldridge/sessions

[#61733120] Use strong parameters in sessions_controller