Timeout session after inactivity

[momo3404]

Fixes sessions not timing out after inactivity.

Changes proposed in this PR:

• Use devise's timeoutable gem to timeout sessions after inactivity.
• Inactivity counts as anything that is not a request to the app. So scrolling, typing, and mouse movement with no requests all count as inactivity.
• After the inactivity period has passed, the user will be signed out once they make a request to the app post timeout and will then be redirected to the login page with the flash message "Your session expired, please sign in again to continue".