Configurable user session duration

[wigledejong]

The time frame of login is short. We use quality time everyday in our stand-up session. But often I'm logged out when I want to change something.

It would be useful to have a longer period, say 5 days that your login is valid or a clear messages on opening Quality-time that your login is expired.

[wkoot]

Add env var SESSION_DURATION to api server, defaulting to 5 days + docs. Check that this is also included by the cookie.
Frontend should not have an own implementation that checks for the duration of the session, only a handler for expired sessions