[#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 containing `:name` and `:password`

(Approach taken from http://edgeapi.rubyonrails.org/classes/ActionController/StrongParameters.html)