Session timeout modal dialog

This adds the modal dialog and countdown requested in #3190.

1. Modal built using Magnific Popup and exposes logic via `modal.wb-session-timeout` event.
2. Countdown of reaction time displayed in modal.  Once zero is reached, session expired message and "Sign in" button displayed.
3. Matched default option property names with 3.1 property names.
4. Fixed indentation of plugin code.

**Todo**

Translations are needed for the following keys:

* %st-timeout-msg
* %st-button-continue
* %st-button-end

**Possible enhancement**

The modal dialog logic could be extracted and made into its own plugin.