Add background ajax query in an attempt to prevent session timeout

[wrongecho]

Whilst this isn't something I've run into myself, we've had a few posts on the forum about sessions timing out when working on things like editing documents.

This PR adds a AJAX background query to keepalive.php every 10 mins which should keep sessions alive.

[wrongecho]

Test these changes at: https://sessiontimeoutgarbageworkaround781.pr-review.itflow.org
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8.0% Duplication