start of clientside session timeout logic
refresh session logic
code and layout for session modal
let showModal accept callback
refresh session with call to heartbeat url; mock implementation of ending client session
add expire_session action to application_controller.  if param redirect=false is appended, then no redirection to root_path takes place
rewrote in JS, finished off, implemented countdown etc.
removed coffee file and test
focus on extend link for keyboard users
remove unused env class, change expire from GET to POST
Merge pull request #305 from ministryofjustice/clientside_timeout

Clientside timeout