Allow setting for UserPresence.awayTime, change default value to 5 sec

[rasos]

When working on the desktop, it takes 5 minutes, until you start receiving push notifications on your mobile. This is hard coded in client/startup/startup.js with
UserPresence.awayTime = 300000;
Our users are missing notifications when leaving the desk. You can not overrule the UserPresence.awayTime with setting your online status to "away" - it still takes five minutes.

We would like to have a config setting for UserPresence.awayTime and we recommend to set the default value to 3000 (= 5 sec.).

The timer seems to start count down, when you are not having the Rocket.Chat browser tab in front or you are inactive on the desktop.

Rocket.Chat Version: 0.56
See also #7115 , #6988 and #1602#issuecomment-226559485

[sampaiodiego]

this is now configurable either on level or server level.