Make user timeout configurable

Make the user timeout configurable

/cc @zendesk/zendesk-radar 

### TODO
 - [ ] :+1: Add more tests for configurable user timeouts

### Steps to merge
 - [ ] :+1: of the team

### References
 - Jira link: https://zendesk.atlassian.net/browse/RADAR-411

### Risks
 - Low: Presence maybe broken.
 - Low: tests may not be valid.
Would be lovely if the test asserted that the time between implicit offline and the offline is > 14 seconds. I think it is possible through looking at p.times I think, where the notification receipt times are recorded.