Session: added missing timeout messages (fixes #3354)

Adds the missing session timeout strings from v3.0.

@pjackson I haven't added the other languages because I have a question about how you want them managed in the i18n.csv Google Doc.  In the 3.0 and 3.1 branches, the `%st-timeout-msg` string uses the word "OK" to reference the button since we can't change the `confirm` dialog's button text.  However, in 4.0,  `%st-timeout-msg`  uses "Continue Session" as the button text.

If you'd like to use the same Google Doc for 3.1 and 4.0, I'll need to split the button text out of the `%st-timeout-msg` string.